In two ways art has rendered assistance to our sense of sight. We stand upon the deck of a ship, while crossing the Atlantic, our eye takes in a considerable prospect of the surrounding waters, the telescope extends this prospect; still, in either case, it has positive limits, which are dependent upon the powers of the eye. This prospect, vast as it seems to us at the time, bears a very small proportion to the real extent of the ocean. Again, bodies soon become divided till their particles are too small for the naked eye to perceive them. That they still exist, and are susceptible of much further sub-division, is rendered certain by the aid which the microscope affords us; we can now follow them with the eye till they are millions of times less than before; but our vision again ceases-we lose the particle-yet we cannot conclude that it has ceased to exist, or ceased to be divisible. There are animals as small as this particle, and the atoms of which they are made up must be considerably less than themselves. The particle we have lost may be capable of further division indefinitely; so that the divisions we can see may bear a much smaller proportion to those we cannot see, than the prospect which the deck of a ship affords us does to the rest of the unseen ocean. Beyond these limits our knowledge of external things cannot extend; they are impassable boundaries. We see how near they approach each other, and consequently how finite our knowledge is. Besides these there are limits of another kind which require to be noticed. They will be best explained, as the former have been, by an example or two. On the discovery of oxygen gas it was concluded by LAVOISIER to be an element necessary to the processes of combustion and acidification; to be the sole supporter of combustion and the sole generator of acids; hypotheses were constructed and the name given accordingly. This was the limit of our knowledge on this subject at the time. A few years later it was discovered that a leaf of copper takes fire spontaneously and burns in chlorine gas, and that hydrogen and chlorine combine and form a powerful acid. Here then was a real extension of our knowledge. If we collect in a strong vessel two volumes of hydrogen and one of oxygen, it is well known that the contact of flame, or an electric spark will cause an explosion, the gases disappear and a drop of water is produced. For some time it was believed that the agency of heat or of electricity was requisite to produce these mechanical and chemical phenomena. But it was afterwards found that if we insert a piece of cold spongy platinum into the mixture, this is sufficient to occasion the gases to explode, and the drop of water to be produced. Thus the previous limits of our knowledge were extended. These examples show that our knowledge of nature has not only a fixed limit, dependent on the powers of onr bodily senses, but that it is also limited by a sliding scale, dependent upon the industry with which we use these powers. This is the boundary which has already so often been extended; these are the barriers which we may still hope to throw down. The small dose of the Homoeopathist, viewed in the light of this double limit, may be thus considered:-chemical tests follow the grain of medicinal substance to the third trituration, that is, till it has been divided into a million of parts, and a good eye, assisted by a powerful microscope, can follow it to the fourth or fifth trituration, beyond this it is absolutely lost to the perception of our sight. The sense of smell can detect musk to the fifth or sixth dilution. Everything that we know forbids us to conclude that the division of matter stops here, but our senses cannot follow it further. On the other hand our power of observing the effects produced by these doses has no limit but that of the sliding scale. Admitting for the moment, what I think I shall afterwards prove, that effects are produced, it is evidently as easy for us to observe them after a dose of the thirtieth as after one of the third or of the first trituration. The same cautions are necessary, but nothing more. Another feature in the character of our knowledge of natural things is our ignorance of modes of action. This also is a result of the very limited powers of our bodily senses. The succession of events can be traced only for a few links, and we cannot discover how even these are connected together. A lucifer match is rubbed on a rough surface and it inflames. How friction produces such a result we know not. If it be said that friction evolves heat, and that heat inflames the match, the question returns, how does friction evolve heat? and how does heat inflame the match? No one can tell. No fact is better ascertained than that the moon is kept in its orbit round the earth, and the earth in its orbit round the sun, by the same force as that which causes a stone or an apple to fall to the ground. These bodies are separated by immense distances, how can they act upon each other? How is it possible for an inert lump of matter to influence another inert lump a hundred millions of miles off? It is by the force of gravitation; but what is gravity? and how does it act? We know not. If we throw a piece of the metal potassium upon ice, it instantly inflames, burns itself into the ice and disappears. Part of the ice has been melted, the water decomposed, its hydrogen burnt, and its oxygen has united with the metal and formed a portion of caustic potash, which is all that remains in the cavity of the ice. These extraordinary phenomena are the effect of chemical affinity, but what is that? and how does it act? No one can inform us. We can surround a seed with suitable proportions of air, warmth, and moisture, and can observe the gradual development of the germ, of the entire plant, and of the ripening seed. How have all these wonderful changes been effected? they are attributed to the vital force, but we know not in the least what that is, nor how it acts. We can examine the various tissues with our microscopes, and analyse them in our laboratories, and thus become acquainted with many new and beautiful facts, which have presented themselves in the course of the growth of our experimental plant. When we have reduced the mechanism to the simplest form, we find that it consists of minute vesicles, formed by an elastic transparent membrane composed of a substance somewhat resembling starch, and called cellulose. When we have obtained the ultimate chemical analysis, we find certain proportions of carbon, oxygen, and hydrogen, with occasionally an addition of nitrogen, sulphur, phosphorus, and a few metals or metallic oxides. We find nothing which reveals to us what vitality is, nor how the successive changes we have witnessed have been brought about. We take food and are nourished; we take medicines and are acted upon by them; we take poisons and die; but how these things act so as to produce such effects we know not. "What is the cause of health? and the gendering of disease? Why should arsenic kill? and whence is the potency of antidotes? If it be said that our food is converted into chyme in the stomach, and into chyle in the intestines, that this is absorbed by the lacteals and conveyed by the thoracic duct into the blood, and that thus we are nourished. I reply, all this is granted, but what then? The question remains as it was, how is all this done? No one can tell. Again, if it be said that medicines act on the nervous system, and stimulate the stomach, that they are sedatives and stimulants, emetics and purgatives, sudorifics, and expectorants; what of all this? What are all these stimulating powers, how do they produce their effects, and how are these effects beneficial? answer is given. No The succession of events,-the steps by which an ultimate result is produced, these, within the limits described, may be observed and experimented upon, but how each step is accomplished is beyond our ken. Of the recesses of nature, of the secret chambers in which her operations are carried on, how forces are “correlative,” how they can be changed into each other, how they act upon matter, how matter acts upon them we are profoundly ignorant. Nevertheless we believe what we see without waiting until we can explain it. Such is the actual condition, the general character and extent of our knowledge of nature, and this consequence follows:-we are not entitled to reject any thing which professes to be a fact, if supported by a sufficient amount of evidence, merely because it is inconsistent with our expectations, does not coincide with our previous opinions, or is not within the limits of our former experience. We are not justified in concluding against a statement of fact by a priori reasoning or theoretical considerations. Analogies may render an assertion probable or the contrary, but no reasoning is conclusive against a matter of fact. The truth or falsehood of the announcement of a fact cannot be settled by reasoning or argumentation, it must be decided by evidence. The case to be stated is this-when a remedy has been chosen in accordance with the law of Homœopathy, (explained in Tract No. 3) an inconceivably small quantity is often a sufficient dose. The difficulty lies in the incredibility of this statement. Be it well observed that the matter in hand is not to account for the efficacy of the small doses, but to prove that they are efficacious. The difficulty is not how to explain their action, but how to believe it. A story is told of the Royal Society, that on a certain occasion it was proposed to that learned body to explain how it was that when a live fish was put into a basin quite full of water, none overflowed. After sundry grave hypotheses had been propounded and objections urged, it was at length proposed to try the experiment. So with this medical difficulty, leaving explanations, let us first try the experiment as a matter of fact. The whole case is embraced by the three questions already proposed. I. Are we acquainted with any facts which render it probable that infinitesimal quantities of ponderable matter may act upon the living animal body? In other words, what does analogy teach us? Look at that bright star! so remote that the astronomer with his telescope cannot calculate its distance, and yet its brilliant beams of light strike upon the eye and convince the merest child of its existence. What a vivid flash that was, and how loud the thunder! See yonder oak riven to its centre,-what an irresistable force, and yet the chemist, with his most delicate balance cannot perceive its weight. Here is a mass of iron, weighing a thousand pounds, moving rapidly upwards, notwithstanding the attraction of the earth to this amount, without any visible link, towards another small bent piece of iron a foot long, encircled with the galvanic current ;-and now falling heavily to the ground the instant that current is arrested. What a mysterious, albeit very visible effect from an invisible, impalpable, imponderable power, generated by such simple means. How warm the fire feels while we stand at the distance of some feet from the hearth! We can imagine how heat will go up the chimney because heated air is lighter than cold air, and will therefore ascend; but how does the warmth get across horizontally to our legs? Oh, it is radiant heat or caloric which travels in right lines in every direction. Very well, but what is radiant heat or caloric? What is light? What is electricity? What is magnetism? Several answers are given by philosophers to these questions. Taking light as the example, there are two modes of explaining it; according to NEWTON, light consists of material particles, emitted by luminous bodies, and moving through space with a velocity of 192,000 miles in a second, and these particles striking the eye produce the sensation of light. According to the other explanation of the phenomena, light consists in an undulating or vibratory movement, which, when it reaches the eye, excites the sensation of light, in the same manner as the sensation of sound is excited in the ear by the vibrations of the air. It is obvious that this theory also presumes the existence of a material medium through and by which the vibrations can be transmitted; in fact it supposes that an exceedingly thin and elastic medium, called ether, fills all space. For our present purpose it is unimportant which theory is regarded as the true one, inasmuch as both assume that matter in some form is concerned in producing the various impressions of light and colour upon the living animal body. The effects are produced by imponderable but not by immaterial agents. To convey some faint notion of excessive minuteness, it may be mentioned that the length of an undulation of the extreme violet ray of light is 0.0000167 of an inch; the number of undulations in an inch is 59,750; and the number of undulations in a second is 727,000,000,000,000 (727 billions); while the corresponding numbers for the indigo ray are, length, 0.0000185 of an inch; 54,070 undulations in an inch; and 658,000,000,000,000 (658 billions) in a second. The other rays differ in similar proportions. "That man," says HERSCHELL, "should be able to measure with certainty such minute portions of space and time is not a little wonderful; for it may be observed, whatever theory of light we adopt, these periods and these spaces have a real existence, being in fact deduced by NEWTON from direct measurements, and in |