APPENDIX 231 



produces the opposite optical effects of various substances. What this mole- 

 cular structure is, was pointed out independently by two observers Le 

 Bel in Paris, and Van 't Hoff in Holland who published their researches 

 within a few days of each other. They pointed out that all optically active 

 bodies contain one or more asymmetric carbon atoms, i.e. one or more atoms 

 of carbon connected with four dissimilar groups of atoms, as in the following 

 examples : 



C 2 H 5 CO.OH 



H-(C) CH a H -OH 



CH 2 .OH CH, CO.OH 



[Amyl alcohol] [Malic acid] 



The question, however, remained do all substances containing such 

 atoms rotate the plane of polarised light ? It was found that they do not ; 

 this is explained by Le Bel by supposing that these, like racemic acid, are 

 compounds of two molecules one dextro-,the other laevo-rotatory ; that this 



FIG. 78. 



was the case was demonstrated by growing moulds, the fermenting action of 

 which is to separate the two molecules in question. Then the other question 

 how is it that two isomerides, which in chemical characteristics, in graphic 

 as well as empirical formulae, are precisely alike, differ in optical properties ? 

 is explained ingeniously by Van 't Hoff. He compares the carbon atom to 

 a tetrahedron with its four dissimilar groups, A, B, C, D, at the four corners. 

 The two tetrahedra represented in fig. 78 appear at first sight precisely alike ; 

 but if one be superimposed on the other, C in one and D in the other could 

 never be made to coincide. This difference cannot be represented in any 

 other graphic manner, and probably represents the difference in the way the 

 atoms are grouped in the molecule of right- and left-handed substances 

 respectively. 



MERCURIAL AIR-PUMPS 



Pfliiger's Pump. I is a large glass bulb filled with mercury ; from its lower 

 end a straight glass tube, m, about 3 feet long, extends, which is connected 

 by an india-rubber tube, n, with a reservoir of mercury, o, which can be 

 raised or lowered as required, by a simple mechanical arrangement. From 

 the upper end of the bulb, I, a vertical tube passes ; above the stopcock, &, 

 this has a horizontal branch, which can be closed by the stopcock, /. The 

 vertical part is continued into the bent tube, which dips under mercury in 

 the trough, h. A stopcock, j, is placed on the course of this tube. Beyond/ 

 the horizontal tube leads into a large double glass bulb, a b ; a mercurial 

 gauge, e, and a drying-tube, d, filled with pieces of pumice-stone moistened 

 with sulphuric acid, are interposed, a is called the blood-bulb, and the 



