De Lautouk. — On a Diatomaceous Deposit. 303 



balsam. If mounted dry they must be mounted on the cover 

 and placed in a cell. Mr. Morland contributed an excellent 

 paper on mounting-media for diatoms to the " Journal of the 

 Quekett Club," August, 1887. And he remarks that diatoms 

 mounted dry cannot be examined under immersion lenses. 

 Another medium — a saturated solution of biniodide of mercurif 

 and iodide of jMtassimn- — owing to its high refractive index, 

 1*68, the highest known in any aqueous solution, gives beauti- 

 ful results, but it is very questionable whether it will last. 

 "The refractive index of this medium is represented by the 

 number 25, as compared with 11 in Canada balsam. In 

 other words, the image is nearly tw^o and a half times as 

 strong."* 



Mr. Morland, however, recommends Canada balsam as 

 the best all-round medium. He also recommends styrax. I 

 find this medium is generally coming into use amongst diatom- 

 ists. It certainly shows up the finer diatoms and the fine 

 markings much better than balsam. It is prepared by getting 

 the ordinary styrax from the chemist, which, by the way, is 

 not true styrax. True styrax has disappeared from com- 

 merce, and is replaced by " Liquidambar orientalis," belonging 

 to the order of the " Altingaceas," or else by " Liquidambar 

 styraciflua," from America. It is very dirty, and for use is 

 prepared by dissolving it in pure benzole or chloroform, filter- 

 ing, and then drying on a plate in a cool oven to the consist- 

 ency of shellac, redissolving in benzole or chloroform, filtering 

 twice, and then evaporating to the proper consistency. To 

 avoid disappointment it is as well to remember that the chlo- 

 roform or benzole must be pure. You will be vexed with the 

 results given by ordinary benzole. " Jackson's " benzole is 

 reliable and the only on6 to be depended on. It is, however, 

 expensive and scarce. 



A mixture of styrax and balsam has been recommended, 

 but I have not tried it. Mr. Morland utterly condemns gum 

 dammar. Other media have been recommended, but in the 

 meantime I would advise Canada balsam or styrax, or a mix- 

 ture of the two. 



To mount the slide, warm it gently, and warm also the 

 cover-glass : put a drop of balsam or styrax, enough for the 

 purpose, over the diatoms, and apply the cover-glass : heat it 

 gently, and examine to see if your specimens are free from air- 

 bubbles ; if not, heat to a greater degree, and while warm tap 

 the slide as for the removal of sand, and you will see the air- 

 bubbles come to the edge. 



Specimens mounted in Canada balsam do not require a 

 ring of cement, but specimens mounted in styrax must always 



* " Microtomists' Vade-mecum," 



