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NATURE 



147 



consciousness of being a dutiful son ; one of a family 

 remarkable for sweetness of disposition and manners, he 

 had lived with his brothers and sisters in terms of mutual 

 love and attachment. He had never lost a friend but by 

 the stroke of mortality, and he felt himself worthy of that 

 constancy of regard. He had followed a profession alto- 

 gether to his taste, and had followed it in a manner, and 

 with a success which procured him the esteem and respect 

 of all competent judges, and set his name among the 

 most eminent, and he was conscious that his reputation 

 was not unmerited ; and with a success, in respect of 

 emolument, which secured the respect even of the igno- 

 rant ; which gave him the command of every rational 

 gratification, and enabled him to add greatly to the 

 comforts of the numerous descendants of his worthy 

 parents — heirs not only of their name, but likewise of 

 their unambitious moderation and amiable simplicity of 

 character." 



Such was Dr. Black as described by those who knew 

 him intimately. We at a greater distance from him can, 

 perhaps, more accurately estimate the character and 

 value of his work. This may be considered under the 

 heads of his three great discoveries, i. The nature of the 

 difference between the mild and the caustic alkalies ; 2. 

 The latent heat of liquids ; 3. The latent heat of vapours. 

 As a student of medicine in this University his attention 

 was early drawn to the chemical characters of caustic 

 potash and caustic soda, the merits of which as remedies 

 in cases of urinary calculus were then much discussed. 



Two kinds of alkalies, the caustic and the mild, had 

 long been distinguished. The former act in a burning, 

 caustic, destructive way on animal and vegetable tissues, 

 the latter do not ; the latter effervesce when mixed with 

 acids, the former do not ; the former are typified by quick 

 . or in slaked lime, the latter by chalk or calcareous earth. 

 Previous to Dr. Black's experiments the difference was 

 thus explained : — When calcareous earth is burnt it 

 becomes quick-lime by taking up from the fii e a fier)', 

 caustic matter ; some of this is given off as heat when 

 lime is slaked, but some of it remains and gives the 

 causticity by which slaked lime is distinguished from 

 calcareous earth. This causticity is transferred (because 

 the caustic matter is transferred) from the lime to other 

 alkalies. Thus, when slaked lime is mixed with a solution 

 of potashes we obtain caustic potash and the lime becomes 

 mild, is re-transformed into calcareous earth, having 

 parted with its canstiaim to the potash. Similarly when 

 sal ammoniac is heated with calcareous earth we obtain 

 sal volatile ; but when we act on sal ammoniac with 

 slaked lime the causticuin passes from the lime to the 

 volatile alkali and caustic ammonia is produced. In all 

 these cases the caustic matter or caustiaim originally 

 obtained from the fire was believed to be transferred from 

 one alkali to another. The effervescence which occurs 

 when a " mild alkali " is treated with an acid was of course 

 observed, but it was looked upon merely as a symptom of 

 the violent movements caused by the mutual saturation 

 of acid and alkali. 



When slaked lime is exposed to the air it gradually 

 returns to the condition of calcareous earth. On the 

 hypothesis stated above, it must therefore gradually give 

 off its causticum into the air. Black's first experiment 

 seems to have been an attempt to catch the causiiacm as 

 it escaped. We have no details of these early experi- 

 ments, but from a note-book which can be shown to be 

 of the date 1752 Dr. Robison extracts the following 

 statement of the result : — " Nothing is given off, the cup 

 rises considerably by absorbing air." Another memo- 

 randum occurs a little later : " When I precipitate lime 

 by a common alkali, there is no effervescence. The air 

 quits the alkali for the lime, but it is not lime any longer, 

 but C.C.C. It now effervesces, which good lime will not.'' 

 A full account of his experiments and conclusions is 

 contained in his graduation thesis (1754), aid in a more 



extended form in 1756 in "Essays and Observations, 

 Physical and Literary, read before a Society in Edin- 

 burgh" — the society which afterwards became the Royal 

 Society of Edinburgh. In this classical paper he shows 

 in the clearest manner that the mild alkalies differ from 

 the caustic by containing in addition a large quantity of 

 " fixed air," a particular kind of gas, which we now know 

 as carbonic acid gas. This gas is given off, causing 

 effervescence when the mild alkali is dissolved in an acid, 

 and the caustic alkali does not effervesce because it does 

 not contain fixed air. Wherever causticity is acquired 

 this fixed air is lost, and vice versa. When slaked lime is 

 mixed with a mild alkali the lime takes the fixed air, and 

 is converted into calcareous earth, while the mild alkali 

 by the loss of fixed air is rendered caustic. In the same 

 way sal ammoniac with calcareous earth gives a mild 

 volatile alkali, the fixed air being transferred from the 

 lime to the ammonia, but with slaked lime a caustic 

 ammonia, because there is here no fixed air to be 

 transfened. The origin of the causticity in the lime 

 is shown to be due to the loss of fixed air which 

 the heat separates from the limestone, and the loss 

 of weight which is observed when limestone is burnt 

 is shown to be exactly accounted for by the loss of the 

 fixed air. Thus Black proved the "causticum" to be 

 minus fixed air. Addition or subtraction of the former is 

 really subtraction or addition of the latter, and transfer- 

 ence of caiistiacm from A to B is really a transference of 

 fixed air from B to A. 



It is impossible to look at such a sketch of this part 

 of Black's work without being struck with the resem- 

 blance between the theory of causticity which he over- 

 threw, the nature of the truth which he discovered, and 

 the method by which he diicovered it, on the one hand, 

 and on the other the theory of Phlogiston, the true 

 nature of combustion, and the method by which it was 

 discovered by Lavoisier ; indeed, Lavoisier himself, in a 

 letter to Black, speaks of the new chemistry as " Une 

 carriere que vous avez ouverte, et dans laquelle nous nous 

 regardons tous ccmme vos disciples." 



The discovery of the latent heat of liquefaction and of 

 vaporisation, was made by Dr. Black while professor in 

 Glasgow. I have occupied so much lime with the purely 

 chemical part of my subject that I shall only here point 

 out : (i) That Black's determination of the latent heat of 

 water agrees very closely with the most recent results of 

 experiments conducted with all the refinements of modern 

 science ; (2) That he studied the fusion and solidification 

 of bodies, such as resin and sealing wax, which pass 

 gradually from the liquid to the solid state, or vice versa ; 

 and (3) That it was his teaching which induced Watt to 

 commence the series of experiments and speculations 

 which led to the discovery of the dependence of the latent 

 heat of steam upon the temperature, and to the invention 

 of the condensing steam-engine. 



A SCHOOL LABORATORY 



FACTS, not theories. This is the special point in the 

 recent "Head Masters' Report" on science teach- 

 ing, and, by his interesting account of it in Nature, vol. 

 xvii. p. 317, Mr. Tuckwell has afforded some opportu- 

 nity of counting the cost to head-masters projecting a 

 development of their science side, but apparently the 

 Report gives no detail, and it is in this direction, perhaps, 

 that many seek chiefly for information ; they would like 

 to know more fully what facilities may be obtained for 

 a particular outlay. 



In the hope of giving reliable information of this sort 

 I submit the following particulars :— 



The governors of Exeter School, hoping to rekindle the 

 torch of science, lately so unhappily extinguished in the 

 West by the Taunton College authorities, have recently 



I. 



