1869.] The Future Water-supply of London. 229 



who require more detailed and trustworthy information may find 

 abundance in Dr. Farr's Cholera Eeport, and also in Appendix 

 No. 11 to Mr. Simon's Ninth Annual Keport; both of which have 

 been abeady quoted. The final conclusion of the best observers 

 on the subject seems to be, that the active agent in cholera is to 

 be found in certain " choleragenic molecules " (they receive different 

 names by different writers), or germs of very low forms of hfe. 

 These molecules are exceedingly minute — not more than asouoth 

 of an inch in diameter — and seem to be indistinguishable in appear- 

 ance, though so very different in power, from the molecules of other 

 zymotic diseases.* They are discharged in myriads in the flux of a 

 cholera, patient ; and hence it probably is that this flux is the main 

 agent in the conveyance of the disease. Dr. Farr proposes to call 

 them " cholrads," and the choleraic matter containing them " chol- 

 rine." There is great convenience in the use of these terms, and for 

 the future I shall adopt them. By careful experiments with this 

 cholera matter Hallier succeeded in cultivating it in various arti- 

 ficial soils, and producing definite forms of fungoid growth. Dr. 

 Thiersch, in 1854, and more conclusively. Dr. Burdon Sanderson, 

 in 1866, made experiments on the action upon mice of papers 

 steeped in cholera flux. Both found that a disease closely resem- 

 bhng cholera could be produced in this way ; and these and other 

 similar experiments seem to leave no doubt that this is the normal 

 mode in which the infection is conveyed. AMiat it is chiefly 

 important to notice here is, that the specific poison of cholera 

 is in all probability not a definite compound, hke arsenic or strych- 

 nine, but a series of extremely minute solid j^articles, each of which, 

 if placed in suitable conditions, may by fissiparity develop into an 

 infinite number. The number which may be communicated to a 

 river by the discharges of a single cholera patient can, of course, 

 only be guessed at. One thousand millions of the choh-ads would 

 not occupy a larger space than a pin's head ; and Dr. Farr gives a 

 curious calculation, founded, for illustration, on the assumption 

 that their number in the cholera flux is equal to that of the 

 globules in the same volume of blood. According to this calcu- 

 lation, a single patient would introduce into the river no fewer 

 than 41,769,000,000,000 of the cholrads. Now, the volume of 

 the Thames at high water, from Bow Greek to Teddington, may be 

 taken approximately as 14,000,000,000 gallons, and each gallon 

 might, therefore, become contaminated with 2983 cholrads If The 



* Beale. To whose marvellously careful observations much of our knowledge 

 on this subject is due. 



t An error occurs in Dr. Farr's calculation, arising from the circumstance 

 that Vierordt's estimate of the number of the blood -corpuscles is copied from the 

 6th edition of ' Carpenter's Physiology,' where it is incorrectly given as 5,069,000 

 per cubic ccntlmclre, instead of per cubic millimetre. I liave introduced the right 

 figures in tlie text. 



VOL. VI. It 



