70 PREPARATION OF DIATOMS. 



for the diatoms thus treated when mounted appear much sharper and 

 cleaner. (') The amount of heat, if the diatoms are rigidly silicious, as 

 most of them are, may be the full power of an ordinary alcohol flame 

 continued for some time, but if they are imperfectly silicious, care must 

 be exercised in the burning. 



" I invariably use old balsam for mounting, just as bought from the 



shops, especially if I wish to have a specimen which will bear 



immediate handling, or be ready to be sent off as soon as mounted. 



Allowing then the cover to cool, while the slide is being cleaned to 



receive it, I place a drop of the balsam, which must not be fluid, 



only viscous, on the middle of the slide, and now with this pick up 



the cover from the little stand where it has been heated. The 



diatoms will be so fastened by the heating, that but few will flow out 



from under the cover, if any, in the subsequent treatment. I now 



hold the slide over the flame of the lamp (which should be much 



smaller than when used for the burning) until not only all under the 



cover is a mass of small bubbles, but until very large bubbles, balsam 



steam, appear ; the flame is removed so soon as the bubbles are 



observed all running to one edge. I press down the cover at this 



place by a mounted pin, and start them in the opposite direction. 



This may seem unnecessary, but long experience shows that this is 



the better way to get rid of them ; during this the slide is held 



somewhat obliquely, the cover is kept from slipping by the pin, and 



if all the bubbles do not disappear, then, with a very small flame, 



heat is applied just beneath the obstinate ones, the slide being held 



slanting, and that part upwards where the bubbles are nearest the edge 



of the cover. The description is longer than the actual process, and 



the slide, when cool, is ready for immediate use. Perhaps I am 



wedded to old ways, but after trial of fluid balsams, without heat, I 



have always come back to the old way ; still, for selected diatoms, 



some of these preparations of balsam are good. If the diatoms are to 



be mounted dry, always the best way, if lor real study, I make a 



ring of the zinc white in balsam ( 2 ) (sold by the opticians) and which 



in a moment or two is sufficiently hard to receive the cover, and 



(') I entirely agree with Professor Smith in his opinion. H.V.H. 



( - ) This cement is made as follows : Some white of zinc boiled in oil is taken and all the oil 

 removed from it by means of repeated washings in benzine. The white paste is slowly dried on a 

 filter paper and then added to a thick solution of Dammar balsam dissolved in benzine. Generally 

 speaking diatoms should be mounted dry as rarely as possible, for it is usually impossible to keep 

 dry preparations without deterioration. They all ultimately become invaded by a mould arising 

 from the varnish. There is, I believe, but one varnish which is suitable for mounting diatoms 

 dry, and which will usually prevent the formation of this mould ; viz., a thick alcoholic solution 

 of gum-lac. H.V.H. 



