EFFECT OF PHYSICAL AGENTS 23 



Fructose was one of the principal artificial sugars pre- 

 pared by Fischer ; it is inactive, but consists of an equal 

 number of molecules of oppositely active sugars termed 

 " Isevulose." One set of these Isevulose molecules turns 

 the plane of polarisation to the right, another set to the 

 left right- and left-handed Isevulose. The left-handed 

 Isevulose occurs in nature, while the right-handed Isevulose, 

 so far as is known, does not. 



Now, on putting brewer's yeast into a solution of 

 fructose, the inactive artificial product, the yeast organisms 

 attack the left-handed Isevulose molecules and convert 

 them into alcohol and C0 2 , while the right-handed Isevulose 

 is left untouched. 



Pressure, unless very great, has little effect on bacteria. 

 Roger investigated the effects of high pressure on certain 

 organisms in bouillon cultures. Pressures of 200 to 

 250 kilos, per square centimetre had no effect ; by raising 

 the pressure to 3000 kilos, per square centimetre one- 

 third of streptococci were killed, and of anthrax without 

 spores a good many ; while sporing anthrax, Micrococcus 

 pyogenes, var. aureus, and the colon bacillus were un- 

 affected. 1 



Our countrymen Downes and Blunt first called attention 

 to the injurious effect of light upon bacteria. If plate 

 cultures be prepared and exposed to sunlight, a portion 

 of the plate being protected from its action, as by sticking 

 on a letter cut out of black paper, and the preparation 

 afterwards incubated, it will be found that the colonies 

 develop at the protected portion only, those parts which 

 have been exposed to sunlight remaining sterile. Although 

 this action of sunlight may occasionally be due to chemical 

 changes in the medium, resulting in the production of 



1 Bacteria being so minute, the actual pressure on a bacterial cell, 

 even with these high pressures, is small. If, for example, a bacterium 

 measures 1 p. by 5 p., a pressure of 1000 kgrm. per square centimetre 

 \vould be but 0-05 grin, (f- grain) on the cell. 



