bnCrystauographtj, 353 



the tongue when brought in contact with it, and a slight, 

 resistance is experienced when we separate them. This 

 eflfect is produced by the facuUy which the body has o,f/ 

 absorbing the saliva which moistens the tongue, and thu8r> 

 bringing the substance into more immeiHate contact with 

 this organ. If wc put a drqp of water on one of these 

 bodies, we shall observe that it is imbibed in an instant, 

 and this proof may suffice instead of applying it to the 

 tongue. . ,M,; bov: 'I. 



8. Colo2trs, In order to fix the degree of confidence ^ 

 which these characters deserve when borrowed from this 

 nrtodification, too much neglected by some, and overvalued 

 by others, we ought to consider it in two very different 

 points of view, according to the various natures of the 

 bodies which are furnished with it. Tn a certain number 

 of these bodies, and in particular in. the earthy and acidi- 

 ferous substances, the colours are owing; to the molecules 

 of a foreign principle, which is frequently iron, and some- 

 times chrome or manganese, disseminated among the mo- 

 lecules peculiar to the coloured body. Hence it happens 

 that one and the same substance, for example fluated lime> 

 is colourless in certain pieces, and in others presents alter- 

 nately the red, yellow, green, violet, &c. In this case the 

 colour is only a transient accident, which may merely serve 

 to distinguish certain varieties. 



But in other minerals, such as metallic substances, sul- 

 phur, amber, some saline substances, the reflection of the 

 rays which produce the colour is made upon the proper 

 parts of the coloured body; it depends on its texture and 

 on the degree of the tenuity of its molecules. It may then 

 he ranked among the specific characters. 



We sometimes find muriaicd soda coloured red. If you 

 dissolve it in this state, it will be stripped of its colouring 

 principle, and the new crystals which it will form will 

 no longer reflect any thing but white light. The operation 

 only frees it from a superfluity, without which it does noti- 

 cease to be of the same nature. On the contrary, sulphated,; 

 copper subjected to the same experiment, as often as we 

 please, will always reappear blue, because this colour i3 

 inherent in its nature. 



Thus those who have said that the true colour of the 

 spinel ruby, for instance, was red mixed with orange, have 

 only designated the sione which pleases amateurs most. , 

 But to say that the true colour of gold is pure yellow, is , 

 to speak the language of the naturalist. If this colour does 

 not uniformly exist in gold, this must arise from the pre- 



Vol, 35, No. 145. May 1810. Z shic« 



