118 



CHEMISTRY. 



[Lesson VIII. 



immerse it in pure water at 60 ; weigh it, 

 and as it weighs less than when in the- air, 

 the opposite pan will fall. Remove weights 

 from the pan a until the equilibrium is 

 restored, and the weight removed will 

 be the weight of the hulk of water dis- 

 placed by the body immersed. Now 

 divide the weight of the body in the 

 air, by the weight of the water displaced, 

 and the quotient will be the density of the 

 body experimented upon, water being =1. 

 We need not be frightened at seeing the 

 specific gravity of any body stated to be 

 9 (as uranium), it only means that it is 



Fig. 22. 



nine times heavier than an equal bulk of 

 water. 



93. T. How can we ascertain the spe- 

 cific gravity of a gaseous body ? 



P. To do this we must have an air- 

 pump. We then take a flask with a stop- 

 cock attached to it, and weigh it when full 

 of air ; this gives the weight of the air. 

 Then we exhaust the air by means of the 

 air-pump, and weigh it again, so as to find 

 the weight of the flask. We have now only 

 to fill the flask with the gas, the specific 

 gravity of which we have to ascertain, and 

 weigh the flask, and then we learn the 

 weight of the gas. For example, suppose 

 that the flask full of air weighed GO grains, 

 and the empty flask 54 grains ; that leaves 

 6 grains as the weight of the air ; and if 

 the flask full of gas weighed 72 grains, it is 

 evident that the gas weighs three times as 

 much as the same bulk of air, or the gas 

 is = 18, and the air = 6. 



9-i. T. How are bodies reduced to the 

 state of powder for chemical purposes ? 



P. By pulverisation, trituration, levi- 

 gation, and granulation. 



<).'). 7'. How are these processes per- 

 formed ? 



P. Pulverisation is the reduction of 

 solid, friable, and brittle bodies, to a state 

 of powder, and is generally performed by 

 means of pestles in mortars. Trituration 

 is performed by a rotatory motion of the 

 pestle, and has the effect of making the 

 powder very fine indeed. Levigation re- 

 sembles trituration, only the process is 

 assisted by using a liquid that docs not 

 dissolve the body operated upon, such as 

 water, spirit of wine, lard, honey, &c. 

 When a painter uses a muller and slab to 

 mix his paints, lie levigates them, (iranu- 

 lation is used to divide mct;ils, and is per- 

 formed by melting and stirring them quickly 

 until cold, or pouring them through a 

 bundle of damp straw, which is shaken 

 while held over a basin of water. 



96. T. What are the means used to 

 separate substances ? 



P. Sifting, washing, or elutriation, and 

 filtration. 



97. T. Explain the processes of sifting, 

 and elutriation. 



p. Sifting is used to separate the finer 

 particles of powders from the coarser, and 

 is usually performed by means of wire or 

 hair- cloth sieves. Elutriation or washing 

 is used to separate the finer parts of 

 powders from the fluids with which they 

 are sometimes mixed. This may be done 

 by using a glass syphon (Fig. 23,) taking 

 care to insert the short leg b into the 

 vessel containing the fluid, until it nearly 

 touches the bottom, then closing the end 

 of the long one with your finger, suck up 

 the fluid by means of the arm c, until both 

 legs are full, and on removing the finger the 

 fluid will flow from a into a vessel placed 

 to receive it. The liquid may also be 

 drawn off by capillary 

 attraction. Sec Q. 

 107, p. 22.] 



98. T. What is 

 filtration? 



/'. It is used to 

 separate solid bodies 

 from fluids, the object 

 being to get a clear 

 liquid. For this pur- 

 pose, take a square 

 piece of white blotting 

 paper, A B c D, (Fig. 

 24), double it over, first 

 at B D, then at A E, and round it with a 



