DAVIES, OX POLARIZED LIGHT. 39 



short time no formation will show itself, but ere long the 

 slide will be well covered, where the wine was laid, with a 

 number of crystals, most of which closely resemble in form 

 the side vicAv of some of the gyrosigmas, but of course are 

 much larger. These crystals are beautiful polarizing objects. 



The next branch of this is the effect of mixing the solu- 

 tions of many of the salts. Here Ave find a new and never 

 ending field for study. The resulting configuration, however, 

 is not fthvays a consequence of any chemical interchange 

 taking place ; it is often simply a mutual interference of form 

 (as it might justly be termed). Still these results are often 

 most interesting and beautiful. Each crystal may contain a 

 portion of the two salts mixed, and yet there may be no 

 chemical combination, as the quantity of each salt may be 

 more or less in proportion to the otlaer ; but this close in- 

 termixture can only take place when the salts are almost, if 

 not quite^ alike as to crA'stalline form. Another modification 

 of these forms takes place when two salts are mixed which 

 do chemically combine, but too great a quantity of one or 

 other being added, on evaporation there is no chance of 

 getting rid of the superfluous portion, and so a granular, or 

 other abnormal, appearance is given to the result, which often 

 greatly interferes witli its beauty for the microscopist. 



Often enough it is not an easy matter to determine whether 

 some polai'izing salts, which have perhaps been accidentally 

 mixed, be formally or chemically joined. Those who pursue 

 this branch for amusement alone might feel little interest in 

 the question, but all who go deeper into the phenomena 

 would feel at least a curiosity to be assured on this point. 

 Sometimes the formation points this out very distinctly by 

 an irregularity not easily mistakable, but in other instances 

 even clever chemists are divided in opinion. 



In preparing these double salts avery obliging agent isoflfered 

 to us in iodine. With many salts this unites, and the results 

 are very fine, giving colours unsurpassed. As an instance of 

 this, we have the preparation of iodo-cyauide of potass, first 

 mentioned, I believe, by an American. This salt is rather 

 unstable, but equal to most objects for the polariscope. 

 Lately I have succeeded in bringing together two salts which 

 I do not know to have been before used for this purpose, 

 viz., the sulphates of copper and magnesia. The peculiar 

 flower-like form of the crystals, centre and petals appearing 

 complete, together with its beautiful polarizing powers, you 

 will yourselves judge of. It is a salt rather difficult to 

 crystallize well, but when once thoroughly understood, the 

 forms seldom differ much. Many have thought this a me- 



