DAVIES, ON POLARIZED LIGHT. 37 



Even a child would be delighted with its beauties, and the 

 highest mind luay here find an unexhausted, indeed, inex- 

 haustible field for both eye and mind. 



It is for sueh a class of ^'^ microscope owners^^ that I 

 venture to offer this notice of " polarized light as a recrea- 

 tion and study." For these men alone I suggest the follow- 

 ing objects ; and if many know all that I Avrite, some fcAv, 

 perhaps, may be ignorant of at least a part. 



CiiYSTALs are generally looked upon as the legitimate 

 field of polarized light ; but man}'- have resigned this branch, 

 deeming it exhausted. There can be, hoAvever, no greater 

 mistake than this. Every day new phenomena start up 

 which we have no means of proving except by this peculiar 

 test. The difference in many crystals (as to form as well as 

 substance), we have no other power of pointing out so simple 

 as this ; and in glass, as well as many other bodies, good and 

 bad qualities lie hidden which are readily tested by this 

 agent. 



Crystals of the same substance, however, are very liable to 

 be mistaken for others, especially by those who are novices 

 in this study. Before we are thoroughly conversant with 

 crystallization we find such a difference in form, even when 

 working with the same salt, that we are inclined to disbelieve 

 in any law governing it. In most instances, however, this 

 is but a seeming difference, as may be easily shown. Sup- 

 pose the true form to be rhombic, some peculiar treatment 

 may be adopted which will so join these rhombs together, 

 cause them to interfere, and ivork into each other, that 

 none but experienced observers can reconcile these apparent 

 anomalies. A very simple proof of this lies in the sulphates 

 of copper or iron. Take a saturated solution of either of these 

 salts, allow it to flow over a glass slide, and dry spontane- 

 ously; the plate will then be found covered with com- 

 paratively broad strips, showing angles of the separate crystals 

 at the edge of each strip. But if, instead of allowing the 

 solution to crystallize untouched, Ave keep it in motion until 

 the liquid is evaporated, the crj'Stals are formed separately, 

 each taking its own peculiar shape, and to an inexperienced 

 eye belonging to a totally different class. I have put up 

 examples of this. One in which the crystals are intimately 

 joined, and the other, where, by simply keeping the solution 

 in motion, they are distinctly formed. 



In anything like chemical inquiry, it is of the greatest im- 

 portance that every agent used should be of the utmost 

 purity. Some of my friends have, however, become pos- 

 sessed of a crystal Avhich they look upon as invaluable, being 



