io6 METHODS OF STERILISATION. 



having been subjected to injury from over-prolonged or excessive 

 heating. This method is known as the intermittent process of 

 sterilisation, and is the only one in use for the preparation 

 of nutrient meat-juice gelatin. The success of this method of 

 killing germs depends on tlie whole of the spores being caused to 

 germinate. Now we know, from statements already made, that 

 there are certain species of bacteria which will only develop under 

 high temperatures, and for whose germination the temperature of 

 the air of a laboratory is therefore insufficiently high. On this 

 account it will be evident that, under certain circumstances, the 

 samples will have to be left to stand at high temperatures. Since, 

 however, these temperatures will, on the other hand, retard the 

 development of the spores of such species as thrive only at low 

 temperatures, it is therefore impossible to neglect either considera- 

 tion. The samples must, consequently, be kept for a certain time 

 at room temperature, and for another interval at higher tempera- 

 tures. In no case, however, must this be relied on without 

 further examination, but it must be laid down as a fundamental 

 rule of conduct, that any nutrient medium apparently rendered 

 sterile by fractional sterilisation, may only be considered as 

 actually sterile, and used as such, when it is found that after a 

 short storage, following the above treatment, no spontaneous de- 

 velopment has taken place. This regulation, urgently necessitated 

 by reason of the insecurity of the sterilising process in question, 

 must not be neglected. Nevertheless, if the work is cleanly done, 

 it will seldom be found necessary to reject samples on account of 

 insufficient sterilisation, since the highly resistant spores, now in 

 question, are generally absent in the majority of the substances 

 employed in the preparation of artificial nutrient solutions, and 

 only creep in when the manipulations are performed without due 

 care. Cultivated soil is rich in such organisms, so that if such 

 soil is, by any means, introduced into these media, an unsuccessful 

 result may readily ensue, as was, for instance, observed by L. 

 HEIM (II.). The occurrence of such spores in meat-extract is no 

 rarity, and the remarks just made should therefore be recalled 

 when such material is employed. 



It may happen that a nutrient medium, which cannot be 

 exposed to a temperature of 100 C. without decomposing, will 

 have to be sterilised. An instance of this is afforded by the 

 solution employed in the study of uric fermentation, which, in 

 addition to the nutrient substances, contains also an admixture of 

 urea. This body, as is well known, is gradually converted at 

 100 C. (in aqueous solutions), into ammonium carbonate. In 

 preparing a medium containing this amide the directions of LEUBE 

 (I.) should be followed, the solution of the other nutrient sub- 

 stances (e.g. a bouillon) being first treated by itself in the steamer, 

 and the urea sterilised separately by heating it in the dry state 

 at 1 06 C. for half-an-hour. By this treatment it is maintained 



