420 Dr. J. B. Sanderson on the Attributes of [Nov. 22, 



that particles which, on development, emerge in organisms so different 

 from each other, possess no structural differences. But if they possess 

 structural differences they must possess the thing differentiated, viz. 

 structure itself." Throughout this passage it is evident that it is not 

 anatomical but molecular structure that is referred to. 



In the other passages which relate to the same subject, I venture to 

 think that Dr. Tyndall has overlooked the distinction made by me 

 between anatomical organization and molecular structure. When, for 

 example, he speaks of " germ structure " in the passage quoted from his 

 Liverpool Address, he evidently refers to molecular structure exclusively ; 

 for he gives ice as his first example, and argues that as ice possesses 

 structure so do atmospheric germs — a proposition which I should not 

 have thought of questioning. 



The experimental evidence which we have before us goes to prove that 

 in all the known cases in which Bacteria appear to originate cle novo 

 (that is to say, in liquids which are at the moment of their origin abso- 

 lutely free from living Bacteria), they really originate from " particles 

 great or small," which particles are therefore germs in the sense in which 

 that word is used by Prof. Tyndall. To illustrate the views I myself 

 entertain, and always have entertained, on this question, I need only refer 

 to my paper on the origin of Bacteria, published in 1871. The experi- 

 ments made by me at that time brought to light the then new fact, now 

 become old by familiarity, that all exposed aqueous liquids, even when abso- 

 lutely free from visible particles, and all moist surfaces, are contaminated 

 and exhibit a power of communicating their contamination to other liquids. 

 As regards water and aqueous liquids in general, I insisted on the " par- 

 ticulate " nature of the contaminating agent, and coined for the purpose 

 the adjective I have just employed (which has been since adopted by 

 other writers), at the same time pointing out that the particles in 

 question were ultra-microscopical, and consequently that their existence 

 was matter of inference as distinguished from direct observation. Dr. 

 Tyndall has demonstrated, by the experiments to which I have already 

 alluded, that the ordinary air also contains germinal particles of ultra- 

 microscopical minuteness. Of the completeness and conclusiveness of those 

 experiments I have only to express the admiration which I, in common 

 with all others whose studies have brought them into relation with the 

 subject, entertain. That such particles exist there can be no question ; 

 but of their size, structural attributes, or mode of development we 

 know nothing. 



Prof. Tyndall, I am sure by inadvertence, has accused me of assuming 

 that there is some relation between the limit of microscopical visibility 

 and what he calls the molecular limit, by which I presume to be meant 

 the size of the largest molecule. Nothing that I have said or written 

 could justify such a supposition. My contention is not that the particles 

 in question are of any size which can be specified, but, on the contrary, 



