ROBERTS' HEAT METHOD. 171 



unacquainted with this property, noticed with astonishment that 

 a development of bacteria occurred spontaneously in vegetable 

 infusions (especially infusions of hay) that had previously been 

 exposed to boiling heat for an hour. The cells were almost ex- 

 clusively in the form of short rods, and were named Vibrio subtilis 

 by Ehrenberg and Bacillus subtilis by Cohn. To obtain them 

 with certainty the following process, known as the heat method, 

 which was devised by ROBERTS (I.), and already referred to in 

 53, is employed : Dry hay is chopped up fine, suffused with 

 a little water, and then left to stand four hours at about 36 C., 

 the usually somewhat acid liquid being afterwards poured off 

 (without filtering), exactly neutralised, and diluted until it shows 

 a density of 1.004. This liquid is then boiled gently for an hour 

 over the sand-bath in a flask plugged with cotton-wool. Since the 

 number of heat-resisting spores present on the hay is frequently 

 but small, sufficient hay and water are taken at the outset to yield 

 at least half a litre after an hour's boiling, which quantity will be 

 sure to contain some living germs. The flask is removed from 

 the sand-bath, left to cool down to the temperature of the hand, 

 and then placed in the incubator, the temperature of which is 

 regulated to about 36 C. during the ensuing twenty-four hours. 

 At the end of this time there will have appeared on the surface of 

 the infusion a thin skin which subsequently thickens and develops 

 to a typical zooglcea. A little of this, examined under the micro- 

 scope, will present the appearance shown at G in Eig. 40, viz., a 

 number of closely adjacent rows of short rods. 



108. Morphology of Bacillus subtilis. 



That, by the aid of the Roberts method, pure cultures (in the 

 present acceptance of the term) cannot be obtained, need hardly 

 be insisted upon, all that is produced being a culture of heat- 

 resisting bacteria; hence the "hay bacillus" prepared in this way 

 by different observers will vary. Really pure cultures may, how- 

 ever, be obtained therefrom by modern methods of pure cultivation. 

 The organism examined and styled Bacillus subtilis by Brefeld is 

 not identical with that of the same name described by Prazmowski, 

 though allied thereto, and indeed so closely that the physiologically 

 important phenomena of spore-germination are alike in both kinds. 

 This circumstance has already been fully noticed in 58, and the 

 reader is therefore referred thereto. A few supplemental mor- 

 phological facts will now be added, the opportunity being also 

 favourable for remarking that a reliable culture of B. subtilis can 

 be prepared by mixing crushed malt and rye in a flask with about 

 four times their volume of water, inserting a plug of cotton-wool, 

 boiling up the mixture, and then leaving it to stand at 35-4o C. 

 A thick, wrinkled skin will rapidly develop on the surface of the 

 liquid, and the contents of the flask acquire a characteristic sickly- 



