28 CHEMISTRY AND PHYSICS IN RELATION 



chemical nor any other theory to explain the communication of the disease ; and 

 it becomes evident that all conditions which are similar to the itch belong to this 

 class, where observation shows approximating or like causes, for the communioa-' 

 tion and extension of the disease. 



CONTAGIOUS DISEASES NOT COMMUNICATED BY ANIMALCULES. 



If now it be asked, what results have been obtained from investigation into 

 these and other similar causes of infectious diseases, we may answer that in the 

 contagion of small pox, the plague, syphilis, scarlatina, measles, typhus, yellow 

 fever, dysentery, hydrophobia, &c., the most attentive observations have not been 

 able to trace any animalcules or organic entities to which the means of propagating 

 the disease could be ascribed. 



PARASITES IN BODIES OF THE HIGHER CLASSES OF ANIMALS, 



We have already observed, that there are a number of insects which can alone 

 be developed and propagated in the body or under the skin of the higher animals, 

 and that they may, in many cases, induce disease, and even death ; and it will, 

 therefore, be perfectly clear that the itch mite belongs to this class of diseases, 

 since the size of the animalcule can make no difference in the explanation. 



There are, accordingly diseases occasioned by animalcules, parasites, which 

 develope themselves in the bodies of other animals, and thrive at the cost of some 

 of their constituent parts ; and they cannot be mistaken for other diseases, where 

 such causes do not prevail, whatever resemblances there may be in external 

 indications. It is possible that further observations may attest the fact, that some 

 or other of the contagious diseases belong to the class dependent upon parasites ; 

 until, however, such a fact be established, we must, according to the rules of 

 natural investigation, avoid assuming it. It is the province of scientific inquiry tc 

 discover the especial causes by which they have been induced, the simple 

 question concerning which will lead the way to an explanation of the subject. 



That infection in contagious diseases is dependent upon an organic being, and 

 that the itch must be regarded as a type of contagious diseases, were facts which 

 it was endeavored to ground upon the deduction of like effects springing from like 

 causes.* A similar mode of reasoning has, for centuries, impeded the advance 

 of the natural sciences, and even continues at the present day to lead to man}' 

 errors. 



The pure miasmatic diseases, and their so-called miasma, have not as yet been 

 laid open to investigation in reference to their origin and the manner of their 

 extension, and on that account no explanation has hitherto been attempted, either 

 by a chemical or parasite theory. The parasite theory has designated muscardine 

 as the type of those miasmatic contagious diseases which arise from matter derived 

 either from the air or from the diseased body. 



MUSCARDINE. 



Muscardine is a disease of the silk worm, occasioned by a fungus. The germ 

 of the fungus, when introduced into the body of the worm, grows in eating its way 

 into the interior, and after the death of the animal it penetrates the skin, when the 

 surface soon appears covered with a forest of fungi, which by degrees dry up, and 

 are converted into dust : this is raised and scattered in the air by the slightest 



*Hen&, Zeitschrift, 2 Bd. p. 305. 



