The disappointment of earlier and more sanguine expectation as to the medicinal value of the protoxide of nitrogen (nitrous oxide gas) has had the effect of withdrawing attention too much from this remarkable agent. That which can work such extraordinary effects upon the nervous system, affecting even the mind with a new sort of inebriety of thought and feeling, must, on the most assured grounds of analogy, be deemed capable of some remedial action, whatever its nature or degree. The inequality of influence upon different persons, and the disagreeable influence upon some, are circumstances common to all powerful remedies we possess, and not fairly to be admitted in argument against its eventual use. Looking to the function of respiration in all its parts, it can scarcely be doubted that cases must occur where it may be important to add to the proportion of oxygen inspired; and the evidence of effects from this particular compound is sufficient to suggest it, under modification, as the most expedient method of attaining the object.

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CHAP. XVIII.

METHOD OF INQUIRY AS TO CONTAGION.

AFTER all that has been written upon contagion, in angry controversy or sober reasoning, I cannot but think that the question still admits of greater precision in the methods of inquiry; whereby to reconcile the contradictions, and remove in some part the difficulties, which still perplex it in so many particular cases.

I take the subject in its most general form; and the term contagion in its broadest application; as implying the communication of disorder in any way from one person to another, without reference to time, or medium of communication. The term infection, being less limited in its derivation than contagion, may be preferred by some; but using the latter word in the wide and popular sense just stated, it cannot well be misunderstood. The particular proofs by which the contagious nature of certain diseases is established, and the enumeration of these diseases, important though the topics are, do not enter into my object.

I believe it may be assumed, that in every instance of communicated disorder of the same kind (except the ambiguous case of certain nervous complaints, where other principles are concerned) there is a materies morbi, a material cause, whatsoever and however subtle is nature and modes of transmission, which is directly concerned in the propagation. This assumption, though large, is justified by observation as far as it goes by strict analogy and probability, where

observation cannot be had. It is likely that future research will rather augment than lessen the number of instances where such miasma or material cause of contagion may be presumed to exist. And it is further probable that we shall hereafter acquire more intimate knowledge of the nature of these morbid matters, of their relations to each other, and the manner in which they are transmitted so as to propagate disease. The course of modern inquiry directs through many channels to these results.

Looking, however, to the principle just stated, and admitting its truth, we shall find basis in it for all reasoning on the laws of contagion; that is to say, for all the conditions which determine and modify the communication of disease from one person to another. A disease is communicated by some morbid matter, thrown off from the first, and capable of producing like symptoms in the second — when conveyed either by inoculation, by simple contact, or indirectly through some medium of transference. Here then three main conditions present themselves, each open to many variations; and, in their combination, capable of producing the numberless varieties and apparent anomalies in the laws of contagion. These are-first, the condition of the person giving the infection; secondly, the state of the person receiving it; and, thirdly, the condition of the medium through which the transference is made. I believe that reflection will show the whole subject to be comprised under these three heads; and that we are bound to refer to them severally in all particular questions or instances which come before us.

The first is very important; inasmuch as it includes all that relates to the varying quantity and intensity of the virus itself. This forms a part of the condition of the patient giving the infection, and the most essential part:-one, how

ever, by no means duly appreciated in the ordinary methods of viewing the subject. Though we have no present method of estimating either the absolute quantity, or the activity of the material cause, or the relation which these conditions bear to one another, this does in nowise lessen the certainty that variations exist in different cases, and have effect, directly or indirectly, in every instance of infection. The modifications arising from the particular period of the disorder, from idiosyncrasies of constitution in those affected, and especially from the variety of the parts or textures through which communication takes place, are all concerned in this view; and have severally their influence in determining the power of the virus, and the course of propagation of the disease.

Under the second head, viz. the state of the person receiving the infection, we have modifications derived from the previous condition of temperament and general health — from the actual health at the time, and particularly from the presence or absence of other specific disorder, counteracting or modifying the virus received and from the state of the organ or tissue first or most intimately affected by it. All these circumstances needfully enter into a just and sufficient view of the subject; but none of them, as I think, are ade quately regarded in our common methods of dealing with it. The latter point in particular has been almost wholly kept out of view, from the great difficulty of reaching it by any distinct proofs; though we cannot for a moment doubt that the condition of the part which habitually, or casually, receives any given infection, must determine the degree of its effects, and probably in many other ways modify their character and

course.

Under the third head, of medium of transmission, still more numerous variations may be presumed to exist. Put

ting aside the obvious cases of inoculation and contact of surfaces, and looking to the atmosphere as the medium in the great majority of instances, we have here the endless variety arising from changes of weight, temperature, humidity, electrical state, and direction of currents - - circumstances ever fluctuating in themselves, constantly changing in their combinations with each other, and capable therefore of modifying infinitely the action of any virus thus conveyed, even without regard to the chemical changes which it may possibly undergo during transmission. And further under this head, we have the case of fomites or virus thrown off from diseased surfaces, imbibed by porous bodies, and again emitted — occasionally, as some assert, in a more concentrated form, from this previous absorption.

Duly considering these several points, they will be found, I think, to show adequate cause for all the strange and perplexing appearances of contagious disease. So far from it being difficult to explain why a given disorder should occasionally appear infectious, at other times not why it should spread rapidly and virulently in some localities, and not at all in others why it should affect some persons, and leave others free-why the cases should be violent at one period, mild at another, it is rather perhaps matter of wonder that the circumstances are not still more varied and irregular than we find them to be. When there are such numerous and ever-changing elements of difference, the combinations of these may well give scope to every assignable variety of result.

It is clear that very many of the contradictions of opinion and statement, as to the contagious nature of certain diseases, may be solved by a reference to these considerations. In all common reasoning on the subject, and even in what has been