PRESERVATION OF ALGJB. 229 



such a growth from taking place, when the object is 

 mounted in water only, provided a germ of one of these 

 minute plants happen to be present, as well as a small 

 quantity of free carbonic acid. 



" My method of making cells in which to mount pre- 

 parations of algJB is as follows : some objects require very 

 shallow, and others somewhat deeper cells. The former 

 may be made with a mixture of japanners' gold-size and 

 litharge, to which (if a dark colour is preferred) a small 

 quantity of lamp-black can be added. These materials 

 should be rubbed up together with a painter's muller, and 

 the mixture laid on the slips of glass with a camel-hair 

 pencil as expeditiously as possible, since it quickly becomes 

 hard ; so that it is expedient to make but a small quantity 

 at a time. For the deeper cells marine-glue answers ex- 

 tremely well, provided it is not too soft. It must be 

 melted and dropped upon the slip of glass : then flattened, 

 whilst warm, with a piece of wet glass, and what is super- 

 fluous cut away with a knife, so as to leave only the walls 

 of the cell ; these, if they have become loosened, may be 

 made firm again by warming the under surface of the slip 

 of glass. The surface of the cells must be made quite flat ; 

 which can be easily done by rubbing them upon a wet 

 piece of smooth marble, covered with the finest emery- 

 powder. 



" When about to mount a preparation, a very thin 

 layer of gold-size must be put upon the wall of the cell, 

 as well as on the edge of the piece of thin glass which is 

 to cover it ; before this is quite dry, the fluid with the 

 cbject is to be put into the cell, and the cover of thin glass 

 slowly laid upon it, beginning at one end ; gentle pressure 

 must then be used to squeeze out the superfluous fluid ; 

 and, after carefully wiping the slide dry, a thin coat of 

 gold-size should be applied round the edge of the cell, and 

 a second coat so soon as the first is dry; a thin coat or 

 two of black sealing-wax varnish may then be put on with 

 advantage, in order to prevent effectually the admission 

 of air into the cell, or the escape of fluid out of it. 



" I would remark, that the gold-size employed should 

 be of the consistence of treacle ; when purchased, it is 

 usually too fluid, and should be exposed for some time m 



