PROCURING OBJECTS. 51 



" Place a drop of water upon a lamina of mica, and put into it of 

 scraped chalk as much as will cover the fine point of a knife, spreading 

 it out, and leaving it to rest a few seconds; then withdraw the finest 

 particles, which are suspended in the water, together with most of the 

 water, and let the remainder become perfectly dry. Cover this re- 

 mainder, so spread out, with Canada balsam, and hold it over a lamp 

 until it becomes slightly fluid, without froth. A preparation thus made 

 seldom fails ; and when magnified three hundred times in diameter, we 

 see that the mass of the chalk is chiefly composed of minute well- 

 preserved organisms." The fossil animalcules in chalk, according to 

 Ehrenberg, amount to twenty-one genera, and forty species. 



50. ANIMALCULES. It is, perhaps, one of the easiest things con- 

 nected with microscopic research, to procure animalcules. In all 

 stagnant waters in the scum of all decaying vegetable infusions in 

 fact s in all stagnant waters containing infusions of organised matters, 

 these animalcules are to be obtained. The surface of infused liquors is 

 generally covered with a thin pellicle which is easily broken, but ac- 

 quires thickness by standing ; the greatest number of animalcules are 

 generally to be found in this superficial film. In some cases they are 

 so extremely numerous, that it is necessary to dilute the infusions ; but 

 this is always to be done with distilled water, and this water should, 

 fov the sake of accuracy, be previously examined with the microscope 

 before it is made use of; the neglect of this precaution has been a 

 source of many errors. To place these minute animalcules under the 

 microscope, the best method is by means of the feeding pin, represented 

 in the annexed diagram. It consists of a glass thread inserted into 



Fig. 32. 



a convenient handle, the end of the glass being enlarged like the head 

 of a common pin, which is to be dipped into the infusion. In this way 

 a small drop of the fluid containing them may be placed on a slip of 

 glass, and covered with a piece of talc, to prevent evaporation, and at 



