18 OF THE FREE DIATOMACEiE, 



get rid of these, the objects to be cleaned should be placed 

 in an evaporating dish of porcelain, or in test tubes (such 

 as are used in chemical laboratories), into which some 

 strong mineral acid — muriatic or nitric — has been pre- 

 viously poured, and the whole suspended over a spirit- 

 1» mp. By this means the fragments of Oscillatorise and 

 other organic matter will be dissolved, leaving a residuum 

 of the silicious coats of the Diatomacese (which are 

 practically indestructible), and also whatever flinty par- 

 ticles of sand may have been introduced : these latter 

 can be got rid of by washing, in a way to be presently 

 described. 



During the boiling which ensues, means must be taken 

 to let the vapour escape that rises from the heated acid, 

 by placing the apparatus, for instance, on the hob of a fire- 

 grate, so that the steam may be carried up the chimney. 

 If this precaution is neglected, serious damage is likely to 

 ensue to articles in the room, the lenses of a microscope 

 will be spoilt, and any metallic substance is sure to be 

 tarnished, not to mention the injury that may be done 

 to the observer himself, from imbibing the poisonous 

 vapour. 



How long the boiling should continue can be determined 

 only by practice and experience ; a few minutes are gene- 

 rally sufficient. Sometimes, however, a second application 

 of acid is needed, before the whole of the organisms are 

 dissolved. The best test, perhaps, of this having been 

 accomplished is the clear bright appearance of the acid 

 that remains in the tube or saucer, and then the Diatoma- 

 ceous valves may be considered as thoroughly cleansed. 

 To clear them of the acid, empty the mass into a tolerably 

 large glass of pure water, and allow the Diatomaceas to 

 sink to the bottom. Immediately pour off the water, 

 taking the utmost care not to disturb the sediment, and 

 introduce a fresh supply, which must, in its turn, be 

 poured off, and the glass refilled. This should continue 

 until a strip of litmus paper ceases to be tinged with a 



