Oct. 17, 1889] 



NATURE 



60 ! 



studied, and the presence of hydrogen in all of these 

 noted. 



Problem XI, To delermine 7vhether oxides stick as 'water 

 and chalk gas may be deprived of oxygen by means of metals. — 

 It being found that hydro<;en and carbon withdraw the oxygen 

 from some but not from all metals, it follows that some metals 

 have a stronger, others a weaker, hold upon or "affinity" to 

 oxygen than has either hydrogen or carbon ; the question arises 

 whether any and which metals have so much greater an affinity 

 to oxygen that they v\ ill withdraw it from hydrogen and carbon. 

 Copi^er and iron have been found to part with oxygen, but zinc 

 and magnesium did not, so these four metals may be studied 

 comparatively. Steam is passed through a red-hot copper tube 

 full of copper tacks : no change is observed. The experiment 

 is repeated with an iron tube charged with bright iron nails : a gas 

 is obtained which is soon recognized to be hydrogen, and on 

 emptying out the nails they are found to be coated with black 

 scale. Zinc and then magnesium are tried, and, like iron, are 

 found to liberate hydrogen. Chalk gas is next passed over red- 

 hot copper, and is found to remain unchanged, but on passing 

 it over red-hot iron or zinc a gas is obtained which burns with a 

 clear blue smokeless flame : this gas is not absorbed by milk of 

 lime, but on combustion yields chalk gas, so it evidently contains 

 carbon, and is a new combustible gas. Like hydrogen, it is 

 found to afford an explosive mixture with oxygen. Finally, 

 magnesium is heated in chalk gas : it is observed to burn, and 

 the magnesium to become converted into a blackish substance 

 unlike the white oxide formed on burning it in air. But it is to 

 be expected that this oxide is produced, and to remove it, as it 

 is known from previous experiments to be soluble in muriatic 

 acid, this acid is added : a black residue is obtained. What is 

 this? Is it not probable that it is carbon? If so, it will burn 

 in oxygen yielding chalk gas. So the experiment is made. 

 These experiments in which hydrogen is obtained from water 

 and carbon from chalk gas afford the most complete "analytic " 

 proof of the correctness of the conclusions previously arrived 

 at regarding water and chalk gas, and which were based on 

 "synthetic " evidence ; taken together, they illustrate very clearly 

 the two methods by which chemists determine composition. 



As hydrogen and carbon form oxides from which oxygen may 

 be removed by means of some metals but not by all, the ques- 

 tion arises. Which has the greater hold upon or affinity to oxygen 

 — carbon or hydrogen ? As it is the easiest experiment to per- 

 form, steam is passed over red-hot charcoal : a combustible gas 

 is obtained which yields water and chalk gas when burnt, so 

 evidently the hydrogen is deprived of its oxygen, and this 

 latter combines with the carbon, forming the combustible oxide 

 of carbon. Will not carbon partly deprive chalk gas of its 

 oxygen ? The experiment is made and it is found that it will. 

 These results afford an opportunity of calling attention to and 

 explaining the changes which go on in ordinary fires and in a 

 furnace. 



Pkoui.em XII, To determine the composition of salt gas, and 

 the manner in which it acts on metals and oxides. — It has 

 previously been demonstrated that spirits of salt or muriatic 

 acid is prepared by acting on salt with oil of vitriol and passing 

 the gas which is given off into water; the solution has been 

 found capable of dissolving various metals and oxides, chalk, 

 lime, &c., and as water alone does not dissolve these substances 

 the effect is apparently attributable to the dissolved gas, so it 

 becomes of interest to learn more of this gas in order that its 

 action may be understood. It is first prepared; its extreme 

 solubility in water is obseived, and also the fact that as it 

 dissolves much heat is given out ; and it is noted that although 

 colourless and transparent it fumes in the air. How is its com- 

 position to be determined ? Is there any clue which can be 

 followed up ? Reference is made to the previous observations, 

 and it is noted that its solution dissolves various metals with 

 evolution of hydrogen ; water alone has no such effect. Is this 

 hydrogen derived from the water or from the dissolved gas? 

 The gas alone is passed over heated iron turnings, and the 

 escaping gas is collected over water : it proves to be hydrogen, 

 so evidently salt gas is a compound of hydrogen with something 

 else. How is this something else to be separated from the 

 hydrogen? Do not jirevious experiments suggest a method? 

 Yes, they have proved that hydrogen has a marked affinity to 

 oxygen, and now it is recollected that on treating muriatic acid 

 with red lead — a substance rich in oxygen — a greenish-yellow 

 gas is obtained. The experiment is repeated on a larger scale 

 and^ the gas is examined. If it is contained together with 



hydrogen in salt gas, perhaps salt gas will be obtained on apply- 

 ing a flame to a mixture of the two gases just as water is from ai 

 mixture of oxygen and hydrogen : the mixture is made and fired,, 

 and the result leaves little doubt that salt gas does consist of 

 hydrogen in combination with the greenish-yellow gas — chlorine. 

 Whence is this chlorine derived — from the salt or the sulphuric 

 acid ? 



The notes are again consulted, and it is seen that a solution 

 of silver in nitric acid gave a characteristic precipitate with 

 muriatic acid but not with sulphuric, so salt solution is added to 

 the silver solution, and a precisely similar precipitate is ob- 

 tained, leaving little doubt that the chlorine is derived from the 

 salt. It is now easily realized that the iron and zinc displace 

 the hydrogen of the dissolved hydrogen chloride. What 

 happens when the oxides are acted on ? In addition to the 

 metal they contain oxygen, which is known to combine readily 

 with hydrogen, forming water ; is water formed ? Lime oxide 

 is therefore healed in hydrogen chloride ; a liquid is obtained 

 which behaves exactly as a solution of hydrogen chloride in 

 water. When the action is complete, after driving off all that 

 is volatile, a solid remains very like fused common salt-^ 

 doubtless zinc chloride, as it is to be supposed that as the 

 hydrogen has taken the place of the zinc the chlorine has taken 

 the place of the oxygen. What, then, is the action of hydrogen 

 chloride on chalk ? It evidently not only separates the chalk 

 gas from the lime, but alo dissolves this latter. What is- 

 formed? Dry (unslaked) lime is therefore heated in a current of 

 hydrogen chloride. It behaves just as zinc oxide, 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 chem'cal 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 are 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 water is driven off, are white solids 

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

 In so 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 new desiccating agents are incidentally 

 discovered. 



Problem XIII. To determine the composition of washing-soda, 

 — The study of this substance is of importance as introducing 

 the conception of an alkali. The preparation from salt is fir.»t 

 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 proportion 

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

 crystallization 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 colour- 

 less, 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 recognized 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 recognized 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 prepara- 

 tion of the metal sodium from soda is then explained. Ac- 

 quaintance being thus made with compounds of chalk gas with 

 two different oxides, the question arises, which oxide has the 



