36 Proceedings of the Royal Physical Society. 
gelatine on cooling; thus each organism is enabled to 
multiply im sitw, and their general appearance, colour, various 
modes of growth can be watched from day to day either 
with or without the use of a lens. The number of spots of 
growth, or “colonies,” as they are called, can be counted; 
and further, from these colonies various other nutrient media 
may be inoculated. In this way each particular set of 
organisms, or colony, can be..studied apart from all the 
others. 
Koch’s plate-cultivation method, though very excellent 
for good drinking waters which have been accidentally con- 
taminated, does not answer for impure or dirty waters, such, 
for example, as may be obtained from many surface wells. 
Such waters, as a rule, contain a number of liquefying 
organisms (2.¢., organisms which liquefy the gelatine as they 
grow), besides various moulds and other fungi, and these 
speedily grow up and choke the typhoid bacilli, Diuluting 
the water abundantly before inoculating the gelatine plate 
does not always give positive results, for by diluting the 
water we scatter further apart the typhoid bacilli, and if they 
are not very abundant the chances that one will obtain a 
colony or growth of them in any particular plate are very 
much diminished. To obviate this overpowering of the 
typhoid bacilli by other non-important organisms, three 
distinct methods have been recently proposed. Briefly, they 
may be denominated—(1) the carbolic or phenic acid 
method; (2) the heat or high temperature method; (3) the 
differential staining method. 
1. The first, or carbolic acid method, was proposed by 
Chantemesse and Widal in 1887.1 To their gelatine for 
plate cultivation they added carbolic acid in the proportion 
of -25 per cent. They maintained that this addition of 
carbolic acid prevented other micro-organisms from growing, 
though it had no such effect on the typhoid bacilli. 
Chantemesse and Widal’s method has been criticised pretty 
severely by Max Holtz? and others, and Holtz has shown 
that a percentage of -25 of carbolic acid is far too high, 
such a strength of the acid effectually preventing the typhoid 
1 Gazette Hebdomadaire, 1887. * Zeit. f. Hygiene, Bd. viii., Heft 1. 
