HISTORY OF DIATOMS. 7 



of alcohol at 90 . Thus the two colouring elements are isolated, and it only 

 remains to evaporate the solvents to obtain the elements in a solid state. 



The plate on the preceding page represents the spectra of various kinds 

 of endochrome which have been obtained by Mr. Petit in a pure state, 

 compared with the spectra of chlorophyl. 



Petit's researches are very important, but according to an anonymous 

 critic, published in a note in the English Mechanic, they are incomplete. 

 At the time when Petit published his work, says the critic, he was 

 unacquainted with the long and important paper published by Sorby in 

 1873 on the colouring materials of the vegetable kingdom. U) 



It is shewn in Sorby's paper that certain colouring materials which had, 

 till then, been considered simple, could be separated into many others. 



Thus the phycoxanthine of Petit, which is identical with the phy- 

 coxanthine of Kraus, consists of the yellow xanthophyl of Sorby with 

 the addition of chlorofuchsine, true fucoxanthine and lichnoxanthine, the 

 whole being slightly stained by the presence of a little chlorophyl 

 (which is itself a compound substance) ( 2 ) and of a very small quantity of 

 phycoxanthine. We must refer the reader who desires to study these 

 different substances to Sorby's paper, as this subject cannot here be 

 exhaustively treated. 



It will be sufficient for the diatom student to know that diatomine 

 is extremely analogous to the chlorophyl of higher plants, and that like 

 it, it decomposes the carbonic acid of the air under the influence of 

 solar light, rejecting the oxygen, and assimilating the carbon. It is in 

 consequence of this action that diatoms are able to perform the impor- 

 tant function of purifying water. 



The valves of diatoms assume every variety of shape and form ; they 

 are, as a rule, mutually symmetrical, slightly convex on the outside and 



(') On Comparative Vegetable Chromatology, by H. C. Sorby. Proceedings of Royal Society, 

 N. 146, June, 1873, pp. 442-483. 



( 3 ) The green colouring matter of higher plants, according to Sorby (op. cit. ), is of very 

 complex composition and that in leaves taken from the shade differs from that in those exposed to 

 the sun. In the leaves of Aucuba Japonica, for example, Sorby found what he called "blue 

 chlorophyl," "yellow chlorophyl," "orange chlorophyl," and "mixed chlorophyl,'' in which 

 latter again he detects " yellow xanthophyl " and "lichnoxanthine." 



The latest researches of the French chemists, Messrs. Fremy and Arm. Gautier, have not 

 pronounced it of so complex a composition ; Mr. Arm. Gautier has shewn that the chlorophyl 

 is a distinct substance, crystallizing in intensely green flat needle-like crystals, which are 

 slightly acid. 



By oxidizing or deoxidising the chlorophyl a variety of derivative salts of yellow, green, red, and 

 brown colours can be formed. When digested in warm concentrated hydrochloric acid, it divides 

 into two substances ; the one which is insoluble in this liquid, but forming a brown solution in 

 warm alcohol or ether, is the phylloxanthine of Fremy ; while the other, which is of a bluish green 

 hue, has acid properties and is the phyllocyanic acid (which he formerly called phyllocyanine) of 

 the same author. 



