PROGRESS IN CHEMISTRY. 251 



by an atom of hydrogen. This represents the solid or stereochemical 

 t'orunda of methane or marsh gas. Now, suppose one of the atoms of 

 hydi'og-en in each of these structures to be replaced by chlorine, the 

 group (OH), or any other monovalent element or group. It is evident 

 that, if not exactly similar (owing to the replacement not having been 

 made at similar corners in each), the two structures can be made simi- 

 lar by turning one of them round mitil the position of the substituting 

 atom or group (which we will term X) coincides in position with X in 

 the stationary one. If two such replacements be made, say with X 

 and Y in each, coincidence can again lie made to take place; but the 

 same is not the case if X, Y, and Z replace the three atoms of h3"dro- 

 gen in the structure; for there is one way of replacement which is the 

 optical image of the other and represents the other's reflection in a 

 mirror, 



X X 



and 



DEVELOPMENT OF BACTERIOLOGY. 



Now, it is found that when the four corners of such a structure are 

 occupied by four separate atoms or groups, or when (as the expression 

 goes) the body contains an '"asymmetrical carbon atom," if the sub- 

 stance of one of its derivations can be obtained in a crystalline form, 

 the crystals are also asymmetric, i. e. , each develops a face which is 

 the mirror reflection of a similar face developed on the other variety; 

 and if a beam of polarized light be passed through the solution of the 

 substance, its plane is rotated to the left if one variety be used, and, 

 if the other, to the right. This hypothesis of LeBel's and Van't Hoff's 

 has had an enormous influence on the progress of organic chemistry. 

 By its means Fischer, now professor at Berlin, has explained the reason 

 of the existence of the enormous number of bodies analogous to grape 

 and cane sugar and has prepared many new varieties; and it appears 

 likely that the terpenes, a class of bodies allied to turpentine, and com- 

 prising most of the substances to which the odor of flowers is due, may 

 thereby find their explanation. It may be mentioned in passing that 

 Pasteur, having found that ordinary mold destroyed one variety of 

 tartaric acid rather than the other in a mixture of the two, and made 

 use of this observation in order to prepare the unattacked variety in a 

 state of purity, was led to study the action of organisms more or less 

 resembling mold; and that this has led to the development of the 

 science of bacteriology, which has had an enormous influence on our 

 views regarding fermentation in general, and guides the work of our 

 physicians, our surgeons (witness Lister's antiseptic treatment), our 

 sanitary engineers in their estimate of the purity of drinking water 



