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the contrary nutritive bodies will be able to diffuse out from 

 the clot into the medium for a long time. It must be admitted 

 nevertheless that the growth which can be obtained on suc- 

 cessful Levinthal agar can hardly be exceeded on any other 

 medium. I have myself once prepared Levinthal agar of 

 excellent quality, but a later preparation did not function nearly 

 so well. I also got just as good a growth on Levinthal 

 agar as on agar containing blood clot (heated to 100°), but the 

 cultures on the latter medium lived the longer. 



The temperature to which different workers heated the 

 blood agar mixture varied from about 70° (the temperature 

 at which the chocolate-brown colour appears) to 100°. Coca. & 

 Kelley and others affirm that it is insufficient to heat it until 

 the brown colour appears but that the full effect is only reached 

 by heating to 96°— 100° for 10 minutes or perhaps rather less. 

 That this is so in every case does not seem to be definitely 

 proved. Soparker (cited from Liston) was satisfied with 

 heating to 66 ; for half an hour (haemolysed human blood) 

 with the results that the solution kept its red colour and 

 transparency (except for a fine, brown precipitate which was 

 filtered off). 



„Chocolate agar" and Levinthal agar are usually prepared 

 by heating the blood after it is mixed with the agar, and 

 several authors lay great stress on this. Olsen and others 

 found that blood heated to 100° together with the agar re- 

 gularly gave better results than when it was heated separately 

 and afterwards added to the agar. This is also in harmony 

 with the older contributions of Pfeiffer, Ghon & Preyss (1) 

 and others that blood heated apart from the agar was not 

 improved as a nutritive substance for Pfeiffer's bacillus but 

 on the contrary became worse. 



Against this Fleming found in comparative, quantitatively 

 performed experiments that it was a matter of indifference 

 whether the blood was heated with the agar or separately. Se- 

 veral American workers (Wollstein (4) and others) adopted 

 the latter method in preparing their media. 



I myself found that blood diluted with distilled water (I 

 used sometimes 1 part blood corpuscles + 4 parts water, and 

 sometimes 1 part blood + 9 parts water) decreased in ac- 

 tivity on heating, while blood (1 part) heated to 100° with 



