242 THE ORIGIN AND PEOPAGATION OF DISEASE. 



namely, that by dilution. The significance of this test depends on the 

 following consideration: If the real vaccine virus be a fluid, it is of 

 course uniformly distributed through all parts of the lymijh 5 and if this 

 lymph be diluted to any extent, the fluid virus will still be equally dis- 

 seminated throughout the whole. When the dilution becomes so great 

 as to extinguish the activity of the virus, this activity ought to dimin- 

 ish and disappear at the same time uniformly through all parts of the 

 liqnid. On the other hand, if the contagious principle reside in the solid 

 particles, each one of which is capable of reproducing its kind, these par- 

 ticles will only be separated from each other by the dilution, and made 

 less likely to be taken up in the drop used for vaccination. But, if one 

 of them should be so taken up, it would still produce its full effect. In 

 this case, the number of successful vaccinations would diminish in pro- 

 portion to the dilution, and the number of failures would increase. But 

 every vaccination which failed would fail completely, and every one 

 which succeeded would produce a normal result. 



Chauveau's experiments showed that the latter supposition was cor- 

 rect. Vaccine lymph might be diluted with from two to eighteen times 

 its weight of water without sensibly losing in efficacy; and in one case 

 the experimenter obtained a single pustule from a number of vaccina- 

 tions made with lymph diluted to j-i^. He obtained, however, the most 

 remarkable results with the lymph of sheep-pox, upon which he experi- 

 mented largely.* He inoculated the same animal, by twenty-one punc- 

 tures, with pock-lymph diluted to g-Jo ; and of these twenty-one inocu- 

 lations eight failed, while thirteen gave origin to full-sized pustules. 

 He then diluted the pock-lymph at once to jooo 5 ^^^ ^^^^ ^^^^ diluted 

 lymph, out of twenty inoculations he obtained only a single pustule, but 

 that pustule presented its normal features, and went through the usual 

 stages of development. 



The active properties of the Ijanph of vaccine and variola, therefore, 

 do not reside in its liquid ingredients, but in its solid corpuscles. These 

 corpuscles, which were already observed by Chauveau and Burdou-Sau- 

 derson. have been recentlj^ examined and described with great care by 

 Dr. Cohn.t This observer adopted every precaution against the intro- 

 duction of foreign elements into the lymph. Some children with healthy 

 vaccine vesicles were brought to the Botanical Institute, the vesicles 

 opened with a new, unused lancet, the lymph taken up by aspiration in 

 a recently-heated capillary glass tube, dropped upon a microscope-slide, 

 and fitted with a glass cover, both the slide and cover having just been 

 thoroughly cleansed with ammonia and boiling water. The edges of the 

 cover were then lacquered down, to exclude the air, and the lymph-cor- 

 puscles examined at successive intervals of time. 



According to Dr. Cohn's observations, these corpuscles are single cells 



* " Comptes Eendus," 1868, tome Ixvii, p. 749. 



t " Organismeu in der Pockeulymplie." ArcMv fur pathologisclie Auatomie uud 

 Physiologie, 1872, px). 55, 229. 



