430 THE MICROSCOPE. 



every care, his specimens are mixed with much foreign 

 matter, in the form of minute particles of mud or sand, 

 which impair their value, and interfere with observation, 

 especially with the higher powers of his instrument. 

 These substances the student may remove in various 

 ways : by repeated washings in pure water, and at the 

 same time, profiting by the various specific gravities of the 

 Diatoms and the intermixed substances, to secure their 

 separation ; but, more particularly, by availing himself of 

 the tendency which the Diatomacece generally have to 

 make their way towards the light. This affords an easy 

 mode of separating and procuring them in a tolerably clean 

 state ; all that is necessary being to place the gathering 

 which contains them in a shallow vessel, and leave them 

 undisturbed for a sufficient length of time in the sunlight, 

 and then carefully remove them from the surface of the 

 mud or water. The simplest method of preserving the 

 specimens, and the one most generally useful to the scien- 

 tific observer, is simply to dry them upon small portions of 

 talc, which can at any time be placed under the micro- 

 scope, and examined without further preparation; and 

 this mode possesses one great advantage, that is, that the 

 specimens can be submitted without further preparation to 

 a heat sufficient to remove all the cell-contents and softer 

 parts, leaving the siliceous epiderm in a transparent 

 state. 



ON CLEANING DIATOMACEOUS DEPOSITS. "The fir^t 

 point to be ascertained is the nature of the material which 

 binds the mass together. In the generality of deposits, 

 this seems to be aluminous or earthy matter, often mixed 

 with some siliceous material which renders the action of 

 acids of little avail. When the bulk of the deposit is 

 clayey matter, the best plan is to place the lumps broken 

 quite small into a vessel and pour on a few ounces of hot 

 water, rendered thoroughly alkaline with common washing 

 soda. This plan frequently answers, causing the lumps to 

 swell, gradually separating into layers, and finally falling 

 asunder into a pulpy mass. The strong soda ley must 

 now be removed by repeated washing, and afterwards 

 boiling in a flask with pure nitric acid ; the whole must 

 afterwards be transferred to a large stoppered vessel and 



