200 



NATURE 



[Dec. 27, 1888 



Ferric chloride 



FeCl, 



5-60. 



FcoClg = 11-19. 



Yellow heat 



1 300- 1 400 

 1 400- 1 500 



Ferrous chloride 



V. Meyer 



FeClo =4-39. 

 Fe2C]4 = 8-78. 



V. Meyer 

 Nilson and Pettersson 



It has been pointed out by Messrs. Friedel and Crafts that the 

 method of Dumas has this advantage over that ot Victor Meyer, 

 that the pressure of the vapour can be accurately ascertained, 

 whereas in Meyer's apparatus diffusion takes place to an un- 

 known extent, so that the actual pressure of the vapour is 

 indeterminable. 



It appears to me that the following conclusions, which are in 

 general accordance with the views of Friedel and Crafts, may be 

 drawn from the results so far obtained : — 



Ahiminium Compounds. — The evidence in favour of the 

 existence of the molecules AI2R6 is overwhelming, but it is 

 probable that at high temperatures they undergo dissociation, 

 thus AlaRg = 2AIR3, 



Gallium Trichloride. — The results point to the existence of 

 Ga^Clg at low temperatures, and GaClj at high, the more 

 complex molecules undergoing dissociation. 



Gallium Dichloride.— Most probably GaClg. 



Indium Chlorides. — Formulas probably InClj, InClg, InCl ; 

 very little indication of the existence of the more complex 

 molecules, but the results are all by V. Meyer's method. 



Chromium Chlorides. — The trichloride CrClj undoubtedly 

 exists ; little or no evidence in favour of CroClg. The di- 

 chloride, even at the highest temperature reached, appears to 

 consist largely of molecules of the formula Cr2Cl4. 



Iron Chlorides. — All the results by Dumas's method, and 

 those of V. Meyer by his own, point to the stability of the 

 molecule FcjClg up to 500° or 600°. The values obtained by 

 Griinewald and Mayer probably point to dissociation into the 

 simpler molecules FeClg, but smce they observed the liberation 

 of free chlorine above 518°, and as the results above 750° are 

 lower even than that calculated for the simple molecule FeCls, 

 there is clearly necessity for caution in drawing deductions from 

 the experiments. 



The values obtained for ferrous chloride indicate, as far as 

 they go, a gradual dissociation of Fe2Cl4 into FeClg. 



Sydney Young. 



University College, Bristol, November 6. 



THE UTILITY OF SPECIFIC CHARACTERS. 



THE question of the utility or inutility of specific 

 characters is one which is of considerable import- 

 ance in the philosophy of biology on account of its con- 

 nection with the action of natural selection ; and it is one 

 which is of special interest at the present time, because 

 of the attention which has been drawn to it by Dr. 

 Romanes's essay on physiological selection, by the Presi- 

 dential address to the Biological Section at the recent 

 meeting of the British Association, and by various letters 

 and articles in Nature and elsewhere. This is a matter 

 upon which a biologist who is practically acquainted with 

 species can alone express an authoritative opinion. It 

 is only the naturalist who has an intimate knowledge of 

 the characteristics and the habits of species who can 

 judge accurately of the relations between such structural 

 features and the animal's habits and surroundings, and 

 who can appreciate the fact that many structures or 

 variations of structure may be of importance, although 

 their precise functions and relations to environment may 

 not yet be known. 



The more minutely a group of organisms is studied, the 

 more the object or utilitarian significance of the specific 

 characters becomes evident. In the Tunicata, the class of 

 animals I happen to have paid most attention to of late 

 years, I am convinced of the practical importance or useful- 

 ness of the recognized specific modifications : — such as the 

 condition of the muscular system, the arrangement of the 

 vessels in the branchial sac, the number and arrangement 

 of the tentacles, and so on — these structures being all 

 related to most important functions, such as respiration 

 and the regulation of the food-supply. Even in the case 

 of such apparently trivial characters as the shapes and 



