OF MICROSCOPIC OBJECTS. 39 



of minute crystals, which cannot otherwise be readily disposed of 

 without the loss of a considerable proportion of diatoms. 



I may here mention that the washing glasses used by Mr. 

 Rylands are stoppered conical bottles varying in capacity from 

 two ounces to one quart ; the conical .form being employed to 

 prevent the adherence of anything to the side ; they are " stop- 

 pered " to render them available in the shaking process about to 

 be described. 



The gathering, freed from acid, is now put into two inches 

 depth of water, shaken vigorously for a minute or two, and 

 allowed to subside for half an hour, after which the turbid fluid 

 must be carefully decanted. This operation must be repeated 

 until all the matter is removed which will not settle in half an 

 hour. The fluid removed should be examined by a drop being 

 put upon a slide, as in some cases very light diatoms have been 

 found to come off almost pure in one or more of these earlier wash- 

 ings. The quantity of water and time of subsidence given may be 

 taken generally, but may require to be modified according to cir- 

 cumstances and the judgment of the operator. By the repetition 

 and variation of this process the shaking being the most im- 

 portant part the gathering, if a pure one, will be sufficiently 

 clean. If, however, it contains a variety of species and forms, it 

 may require to be divided into different densities. 



In some cases, however, it is best to divide the gathering as a 

 preliminary operation, which may be done by agitating it in a 

 quantity of water and decanting what does not readily subside. 

 The heavier and the lighter portions are then to be treated as two 

 separate boilings. But when the cleansing has been carried to 

 the above stage and this division is required, the plan must be 

 somewhat as follows : The gathering must be shaken in a test- 

 tube with six inches of water, and then allowed to subside until 

 one inch at the top remains pure. About three inches are then 

 to be carefully withdrawn by a pipette, when the tube may be 

 filled up and the operation repeated. The three lower inches also 

 may then be decanted and examined. The gathering is thus 

 divided into three portions, viz. that which was withdrawn by 

 the pipette, that which remained floating in the lower three inches 

 of water in the tube, and that which had settled at the bottom. 



