30 DAVIES, OX POLARIZED LIGHT. 



chanical mixhire only, but I think it is certainly chemical. 

 One of our standard works on chemistry says, " Of the five 

 atoms of water wliich sulphate of copper contains, one is con- 

 stitutional, and may be replaced by the alkaline sulphates to 

 form a class of double salts of great beauty/'"^ I have slides 

 of this. 



In mentioning iodine above as an '' obliging agent," I 

 omitted to state, that very often 1 have thought the salts 

 which I have been crystallizing gave better defined forma when 

 used with a small quantity of iodine, although it certainly 

 did not enter into any chemical combination with them, not 

 altering in the least the general characters of these crystals. 

 Has any other person worked in this direction who would 

 give his experience ? 



In working amongst these crystals there is but one draw- 

 back, and that is the instability of some of them. The diffi- 

 culty of setting them is very great, as some will change, 

 whether set in balsam, dry, or in oil ; but this only occurs 

 Avith a few, and the beautiful resiilts of many, more than 

 compensate for the difficulties offered by a few. 



With the crytals a novice thinks the field of the polarizer 

 is well nigh exhausted. He can make no greater mistake 

 than this ; its field is no more limited than that of the mi- 

 croscope without a polarizer. Many of the Zoophytes 

 which he can pick upon any sea-shore will surprise him when 

 used with this agent. Some of the Cellulariadse, Gemellariae, 

 &c., afl^brd beautiful specimens of this; many of them 

 appearing to be golden casts from the richness and brilliancy 

 of their colours. Some of this class, however, are irreme- 

 diably opaque; but others, when set in balsam, are quite 

 transparent enough to use with the power they require or 

 even admit of. 



I have alread}^ written enough, perhaps, on this subject ; 

 but I should be omitting one of its great functions if I made 

 no mention of the Foraminifera as well adapted for pur- 

 poses of polarization. Here is a great field for research ; 

 and even as objects of show we may do much work. Amongst 

 the most transparent we find many specimens, which almost 

 resemble miniature rainbows when viewed by polarized light, 

 more especially when set in balsam. Of many which are too 

 opaque we must make sections, but we shall be repaid for 

 our trouble. Some of the minute molluscs give us a cross 

 closely resembling that produced by starch, though not quite 

 so well defined. Amongst the sponge spicula we may also 



* Dr. Kane's 'Elements of Chemistry,' article, "Sulphate of Copper." 



