ON TEACniNO CHEMISTRY. 245 



yielding a liquid product— evidently a solution of hydrogen chloride in 

 water, as it dissolves zinc -with evolution of hydrogen, and the residue is 

 like that of zinc chloride. The important discovery is thus made that 

 lime also is an oxide— that chalk, in fact, is a compound of two oxides ; 

 the resemblance of lime to zinc oxide and magnesium oxide is so striking 

 that the conclusion is almost self-evident that lime is probably a metallic 

 oxide, and it may be here pointed out that this actually is the case. The 

 gradual discovery of the composition of chalk in the manner indicated is 

 an especially valuable illustration of chemical method, and serves to show- 

 how chemists are often obliged to pause in their discoveries and to await 

 the discovery of new facts and methods of attack before they arc able to 

 completely solve many of the problems which are submitted to them. 

 The solids obtained on dissolving zinc oxide and lime in muriatic acid 

 and boiling down the solution, when all the Avater is driven off, are white 

 solids like fused salt, but on exposure they gradually become liquid. In 

 60 doing they increase in weight, and evidently behave like sulphuric 

 acid. Probably water is absorbed from the air : no change takes place 

 when they are kept over sulphuric acid or dry lime. In this way two 

 now desiccating agents are incidentally discovered. 



Problem XIII. To determine the composition of waslmig soda.~-The 

 study of this substance is of importance as introducing the conception of 

 an alkali. The preparation of washing soda from salt is first described. On 

 heating the crystals they melt and give off 'steam' ; the experiment is made 

 in such a way that a quantity of the liquid is obtained sufficient to place 

 beyond doubt that it is water. The water is found to be easily driven off 

 on heating the crystals in the oven, and to constitute a very large pro- 

 portion of the weight of the crystals. The conception of water of 

 crystallisation is thus gained. On heating the dried substance to full 

 redness in the platinum dish, no loss occurs. The residue dissolves in 

 water, and ' soda crystals ' may again be obtained from the solution, so 

 that heat does not affect it. Perhaps acids which have been found to act 

 so powerfully in other cases will afford some clue— on trial this is found 

 to be the case : a colourless, odourless gas is given off, which extinguishes 

 a burning taper. Is this perhaps nitrogen or chalk gas ? The lime- 

 water test at once decides that it is the latter. So it is determined that 

 •washing soda, like chalk, is a compound of chalk gas — but with what? 

 With an oxide ? The dried substance is heated in hydrogen chloride : 

 chalk gas is given off as before, and a liquid which is soon recognised as 

 water saturated with hydrogen chloride. The residue dissolves in water, 

 and separates from the concentrated solution in crystals exactly like salt, 

 and, in fact, is soon recognised to be salt ; evidently, therefore, that 

 ■which is present in salt along with chlorine is present in soda crystals 

 along with oxygen, chalk gas and water. The preparation of the metal 

 sodium from soda is then explained. Acquaintance being thus made 

 with compounds of chalk gas with two different oxides, the question 

 arises, which oxide has the greater affinity to the chalk gas ? Will lime 

 displace sodium oxide from soda or rice versa ? On adding lime water 

 to soda solution, a precipitate of chalk is formed. What does the solu- 

 tion contain ? Lime water contains lime in combination with water ; is 

 the sodium oxide present in combination with water ? Soda is boiled 

 with milk of lime (in an iron saucepan to avoid breakage) until it no 

 longer affects lime water; afterwards the liquid is poured off and boiled 

 down. The product is very unlike soda : it is very caustic, and when 



