H. MORLAND ON MOUNTING MEDIA FOR DIATOMS. 113 



by styrax. I should be disposed to give the palm to phosphorus, 

 it having an extremely high refractive index. I have never 

 worked at it myself, but from all accounts it is very difficult of 

 manipulation ; it requires being dissolved in bisulphide of 

 carbon, but no more of this must be used than absolutely 

 necessary, as otherwise the high refractive index is considerably 

 reduced. Complaints have been made about phosphorus slides 

 not being permanent, but, on the other hand, I believe that Mr. 

 Stephenson, who first made use of this medium, has perfect 

 phosphorus slides in his possession which have been mounted 

 for some years past. Prof. H. L. Smith's and Dr. S. Meate's 

 media are both of the same class, having the elements of 

 sulphur, arsenic, and bromine in each ; slides newly-mounted in 

 these media are superb, but from what I have heard, and from 

 my own experience of Dr. Meate's compound, I should say that 

 neither of them is to be relied upon ; sooner or later a granu- 

 lation sets in, and it is only a question of how long it will be 

 before the whole of the mount is thus disfigured. I believe that 

 some microscopists may be found who may be disposed to think 

 that some slides mounted in Dr. Meate's medium may prove 

 permanent, to which I answer that the large number of failures 

 is more than a sufficient reply ; a medium is not to be depended 

 upon unless you can rely upon having a larger proportion of 

 permanent slides than, say, one out of a dozen mounts. It has 

 struck me that possibly the non-permanency of these two media 

 is due to the evaporation of the bromine, and that if a proper 

 cement were found for confining the same, they could then be 

 used with every confidence in their efficiency. 



Prof. H. L. Smith has also introduced two other chemical 

 media with high refractive indices, but lower than those just 

 referred to. Of one, called " Stannous Chloride Medium," I 

 know little or nothing, and am unable to say whether it has 

 proved permanent or not ; the other medium, a preparation of 

 antimony bromide and boro-glyceride, is in my hands, as well as 

 in those of a correspondent of mine in America, a distinct 

 failure ; at first, all was superb, but after a short time crystals 

 began to make their appearance and gradually spread all over 

 the mount, completely spoiling it. 



I have seen an account of another chemical medium, viz., 

 sulphur in aniline, and thought I would try my hand on same, 



