114 



CHEMISTRY. 



[Lesson VII, 



78. T. Then this knowledge is practi- 

 cally applied .' 



P. Certainly ; in laying down railways, 

 building iron bridges, and constructing all 

 works of the kind where metal is much 

 employed, provision is made for expansion ? 



/'. How do people know how much 

 to calculate for the expansion ? 



P. By referring to tables of the ex- 

 pansion of various bodies, and remember- 

 ing that the degree of expansion is in the 

 direct ratio of the increase of temperature, 

 and that when the heat decreases, the body 

 will contract again to its former size. 

 MM. Lavoisier and Laplace investigated 

 the subject, and the following is the 

 result : 



linglish flint glass ......... T^CS* 



Common French glass .. 



Glass without lead 



Steel, untempered 



Steel, tempered 



Soft iron 



Gold 



Brass 



Silver ........................ T , 



. Lead ....................... *iT 



80. T. What causes liquefaction ? 



P. The repulsive agency of the caloric, 

 which drives the particles of bodies capable 

 of assiimirig the liquid form so far apart, 

 that their cohesion is diminished, and they 

 are rendered easily movable on one another 

 in any direction. 



81. T. What is vaporisation ? 



P. The rapid conversion of a solid or 

 liquid into an aeriform state, as when water 

 is converted into steam by boiling it Now 

 \vater boils at 212 under ordinary atmo- 

 spheric pressure, but when this is removed 

 it boils at a lower temperature. Steam is 

 invisible, colourless, and transparent, which 

 may be proved by looking at the spout of 

 a tea-kettle, when it will be found that the 

 first half inch from the spout appears 

 unoccupied, but that the rest of the space 

 in front is occupied by what is generally 

 called steam, which is in reality condensed 

 vapour. Steam is 10% times greater in 

 bulk than water; therefore, one volume of 

 water will yield 1696 volumes of steam. 



* The fractions show the amount of expansion 

 in length of the rods of the various bodies passing 

 from 32 to 212. 



82. /'.What is evaporation ? 



P. The dissipation of a liquid by, jlj", 

 conversion into vapour. It may be spon- 

 taneous, or caused by the application < 



83. T. How is spontaneous evap 

 produced ? 



P. Partly by the tempcratur 

 partly by the solvent power of the atnu*. 

 spheric air forming a solution of the body. 

 Ether, alcohol, and volatile oils ; 

 amples of the class of bodies that undergo 

 this change. 



84. T. What is the object of cm] 

 heat to cause evaporation ? 



P. To drive off the fluid from the sub- 

 stance held in solution ; for, evaporation 

 is generally used for the purpose of obtain- 

 ing salts from aqueous solutions ; b 

 when the fluid is valuable, as alcoi- 

 example, then the process of distillation is 

 employed. When we wish to evaporate 

 any solution, we employ a ~stand like that 



fig. 1C. 



in Fig. 16, which has a movable slide (a), 

 so that we can raise or lower the evaporating 

 basin (&); the heat is supplied by means of 

 an argand gas-burner, (e), supported on a 

 heavy base, and furnished with a flexible 

 tube of vulcanised India-rubber (d). If 

 gas cannot be procured, a common '.argand 

 oil-lamp, or a spirit-lamp will do. 



85. T. Is it necessary to employ a 

 great heat for evaporating purposes, such 

 as you have described ? 



P. No. A very convenient method is 

 to use an iron saucepan filled with sand, 

 which is heated by resting it upon some 



