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only from different agents, but in different ways as regards the organs first affected; and with varying influence on different parts of the system. The agency of the hydrocyanie acid (an important addition to this class of medicines, and perhaps the most explicit example of their effect) is evidently not the same as that of digitalis or tartarised antimony; and we have equal reason to suppose differences between the latter, seeing the diversity of their collateral effects. Though greatly instructed by modern inquiry in the action of poisons, yet is our knowledge likely to be much augmented in all that concerns their relation to the vital properties of the blood. And we have the assurance of great aid in the treatment of disease, by better determining the specific nature and application of those agents which are directly opposed to excess of action, either throughout the whole system, or in particular parts and functions of life."

* In alluding to the very powerful medicines of this class, and particularly to the prussic acid, it may be worth while to make one comment; needless, indeed, to all who are familiar with the principles of therapeutics; but requisite to be kept in view while these agents are still waiting, as it were, for recognition, and deprecated by many who see in them only the virulence of their concentrated forms. What we have mainly to regard, in estimating the medicinal value of any substance, or its just application to practice, is the well defined nature of its action on some organ or function of the living economy. If this action be clearly ascertained, we have essentially a medicinal power in our hands. Every such agent, even the most simple, is capable of being misused by excess ; and this excess, or the fitness of use, is determined, not by any comparison of the power of different agents, but simply by the amount of the effects appropriate to each. The prussic acid, diluted as befits the particular application given to it, is not, in any practical sense, a stronger medicine than others most familiar to us, nor more dangerous in use: and we have even some additional security in the more definite nature of its effects, and in the greater care bestowed on its administration.





I PUT this title interrogatively, as expressing what is merely a speculation; and, in so far, distinct from most of the other topics of this volume; yet sufficiently within the scope of possible truth to justify a few remarks a few remarks upon it. In making these, the reasoning may conveniently be applied as an argument for the hypothesis; viewing it constantly, however, under the qualification just named.

The question is, what weight we may attach to the opinion that certain diseases, and especially some of epidemic and contagious kind, are derived from minute forms of animal life, existing in the atmosphere under particular circumstances; and capable, by application to the lining membranes, or other parts, of acting as a virus on the human body? This is by no means a new speculation; and it would seem to have been much more frequently started during the last century, than at the present time. The greater exactness of modern


* Kircher is known as one of the earliest propounders of the opinion. Linnæus gave his sanction to it, by inserting in the Amoenitates Academicæ several memoirs on the subject. The most detailed is that under the name of Nyander, entitled "Exanthemata Viva," in which small-pox, measles, the plague, dysentery, syphilis, and hooping-cough are all attributed to the agency of minute animals, chiefly Acari of different species. A second paper in the same work, on Lepra, applies the speculation also to this disease; and other memoirs, severally entitled "Mundus Invisibilis," "Miracula Insectorum," and "Noxa Insectorum," produce the hypothesis in a more general form.

inquiry rightly represses all opinions, which have not explicit facts for their support. Though the course of discovery has recently been approaching, in some points, nearer to the hypothesis in question, it still furnishes nothing beyond stronger presumptions and more numerous analogies; nor has any endeavour been made to collect or class these, with a view to more general results. Nevertheless the subject is one fairly open to inquiry; and the more reasonably so, as we possess no information regarding the causes of these maladies, which can supersede research, but rather have in our ignorance the motive for pursuing it through every new channel which science may disclose.

That there are conditions of animal life in the atmosphere (however characterised), as minute, as numerous, and as variously diffused, as those of which the microscope informs us in water and other media, may be considered from analogy next to certain. Our actual knowledge carries us so far into these minute forms of existence, and by such uniform gradations, that we cannot suppose the series to stop, because evidence is no longer drawn from our own senses, or means of research. This would imply a sudden breach of continuity, such as we find in no other part of the scale of animal being. It is only of late that the wonderful eye of the microscope has clearly disclosed to us that vast domain of life to which the infusoria belong;-a new world of organised and active beings, which, but for the access thus afforded by the happy artifice of a single instrument, might have remained for ever as much hidden from our sense and knowledge as the invisible forms of insect life, of which the hypothesis before us presumes the existence.*

* Even the unaided eye, however, gives us certain probable notices of these swarms of minute beings, which escape all individual examination. Reaumur and other naturalists have conjectured that the small floating


Whether we may hereafter reach more direct evidence on this subject is yet uncertain. Out of the direct dominion of the microscope these animals are removed, unless some such method as that suggested below be found attainable.* Other means, however, are conceivable, seeing the number and variety of resources furnished by modern science, and the unexpected quarters from which knowledge is often derived. I may name as an instance of this the paper of Dr. Wollaston (Phil. Trans. for 1820), "On Sounds inaudible by certain Ears;" showing the probable existence of whole domains of insect life, capable of exciting vibrations in the air, of which man's grosser hearing is wholly unconscious; but which, received by their finer organs as audible sounds, minister to purposes of enjoyment and activity among beings unperceived by any of the human senses.

Even admitting, however, that we may never reach actual

clouds (nubeculæ ætherea), like more opaque portions of the atmosphere, which, under summer temperature and in certain lights, are seen near the surface of the earth, are in fact insect swarms, depending for their fugitive existence on the conditions of the medium around them; but in this, as well as in their living habitudes, resembling the insect species more obvious to us.

*It has been supposed that the collection and condensation of dew, in situations where malaria or infectious miasmata abound, might afford a possible means of subjecting these material agents (for such they doubtless are) to chemical or microscopical research. If it were true, as presumed in the hypothesis before us, that minute insects are concerned in giving pestiferous quality to a portion of air, might not colouring matter, suitable to animal organisation, be applied to the natural or artificial dew condensed from this air, so as to afford the chance of similar success to that which Ehrenberg has attained in his researches on the infusoria? The absence of any such observation, when he was himself seeking to discover infusoria in dew, does not disprove the possibility of this; as, from the difference in the cases presumed, the results, if ever thus obtained, would probably be only partial and occasional.

proof of these more minute forms of life, be they insect or of other kind, the probability of their existence is little lessened by the failure, seeing the obstacles which produce it. And if existing, the same analogy, carried further, will lead us to other not less probable inferences regarding the habits and instincts, in which they may be presumed to have affinity with the known insect genera. Such are, their frequent sudden generation, at irregular and often distant periods, under certain circumstances of season or locality, or under other conditions less obvious to apprehension: - and the diffusion of swarms, so generated, and with rapidly repeated propagation, over wide tracts of country, and often following particular lines of movement. The further inference which more especially concerns our subject, rests also on analogy, though of less explicit kind, viz. that certain of these animalcule species may act as poisons, or causes of disease, upon particular parts of the body exposed to their influence.

These presumptions, supported by various reasons, and contradicted by no ascertained facts, give foundation to the question before us. Whatever is true or peculiar as to the habits of insects obvious to our senses, is likely to be equally or more so in those whose minuteness removes them further from observation. Their generation may be presumed more dependent on casualties of season and place; — their movements determined by causes of which we have less cognisance; and their power of morbidly affecting the body to be in some proportion to their multitude and minuteness. It cannot be deemed too mechanical an idea, that all conditions which give readier and more extensive access to the internal membranes, to any species capable of acting as a virus on these parts, may increase their influence as causes of disease. It occurs then, as the first point of inquiry, whether we have grounds for believing that animal life under this form can

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