106 



CHEMISTRY. 



with a pressure of 400 atmospheres. The tem- 

 perature at which liquid ceases to be formed 

 under any degree of pressure that can be ap- 

 plied, is called the " critical point." For car- 

 bonic acid this point is 30.9 C., or 87.7 Fahr. 

 Prof. Andrews conceives these experiments to 

 afford the strongest grounds of believing that 

 the liquid and gaseous conditions are only 

 different forms of the same state of matter. 



A Law i)i Chemical Dynamics. Messrs. 

 Gladstone and Tribe communicated to the 

 Royal Society in June the results of their 

 investigations of the process by which one 

 metal iias the power of decomposing another 

 metal, until the more powerful metal has en- 

 tirely taken the place of the other. Their 

 experiments were generally performed as fol- 

 lows: In a tall glass they placed 72 cubic cen- 

 timetres of an aqueous solution of a salt of 

 known strength, at 12 C. ; a perfectly clean 

 plate of metal of 3.230 square millimetres was 

 then weighed and placed vertically in this 

 solution without reaching either to the top or 

 the bottom ; the action was allowed to proceed 

 quietly for ten minutes, when the plate was 

 removed, and the deposited metal scraped off. 

 The loss of weight gave the amount of metal 

 dissolved, and represented the chemical action. 

 The most complete series of results was with 

 copper and nitrate of silver : 



la the earlier terms of this series, twice the per- 

 centage of silver-salt gives three times the chemical 

 action. The close agreement of the observed num- 

 bers with those calculated on this supposition con- 

 tinues as far as the 9th term. The law_ then breaks 

 down, and after about 7 per cent, the increased ac- 

 tion is almost in direct ratio with the increased 

 strength. The position of the plate in the solution 

 was found to make no difference to this 2-3 law. 

 Similar series of experiments were made with zinc 

 and chloride of copper, zinc and sulphate of copper, 

 zinc and nitrate of lead, iron and sulphate of copper, 

 and other combinations ; and, in every instance where 

 the solution was weak and the action simple, the law 

 of three times the chemical charge for twice tho 

 strength was found to hold good. It was proved 

 that the breaking down of the law at about 3.5 per 

 cent, of salt in solution was irrespective of the quan- 

 tity of the liquid, or of the time for Avhich the plate 

 was exposed, with 72 cub. centims. of a 1.41 per 

 cent, solution of nitrate of silver, the rate of action 

 remained sensibly the same for as long as twenty- 

 five minutes, notwithstanding the constant deposi- 

 tion of silver. This apparently paradoxical result is 

 due to fresh relays of the original solution being 

 brought tip to the plate by the currents produced, 

 and that period of time elapsing before any of the 

 products of decomposition are brought back again 

 in their circuit. 



When it was perceived that within easily-ascer- 

 tainable limits the chemical action is the same for 

 pimilar consecutive periods of time, experiments were 

 made in far weaker solutions. It was only necessary 

 to lengthen the time of exposure. It was thus found 

 that the law of three times the chemical action for 

 twice the strength of solution holds good through 

 at least eleven terms of the powers of 2 ; in fact, from 

 a solution that could dissolve one gramme of copper 

 during the hour, to a solution that dissolved only 

 0.000001 gramme, a million times less. 



The manner in which the silver is deposited on a 

 copper plate was examined, and the currents produced 

 were studied. At first a light-blue current is per- 



ceived flowing upward from the surface of the plate, 

 presently a deep-blue current pours downward, and 

 these two currents in opposite directions continue to 

 form simultaneously. A similar phenomenon was 

 observed in every case where a metallic salt attacked 

 a plate of another metal. The downward current 

 was found to be a solution of almost pure nitrate of 

 copper, containing about three times as much N0 3 as 

 the original silver solution, while the upward current 

 "was a diluted solution of the mixed nitrates. More- 

 over, the heavy current took its rise in the entangled 

 mass of crystals right against the plate, while the 

 light current flowea from the tops of the crystalline 

 branches. It was evident that, when the fresh silver 

 was deposited on these branches, and the fresh cop- 

 per taken up from the plate, there was not merely a 

 transference of the nitric element from one combina- 

 tion to another, but an actual molecular movement 

 of it toward the copper plate, producing an accumu- 

 lation of nitrate of copper there, and a corresponding 

 loss of salt in the liquid that is drawn within the in- 

 fluence of the branching crystals. Hence the oppo- 

 site currents. 



The amount of action in a circuit of two metals and 

 a saline solution must have as one of its regulating 

 conditions the conducting-power of that solution. It 

 appeared by experiment that a strong solution of 

 nitrate of silver offers less resistance than a weak 

 one ; and it was also foundj on adding nitrate of 

 potassium to the nitrate of silver, that its power of 

 attacking the copper plate was increased ; that tho 

 augmentation of the foreign salt increased the action 

 still further; and that the 2-3 law holds good be- 

 tween two solutions in which both the silver and 

 potassium salt are doubled, though it does not hold 

 good if the quantity of foreign salt be kept constant, 

 bimilar results were obtained with mixed nitrates 

 of silver and copper. 



Heat in Chemical Combinations. Dr. An- 

 drews, of Belfast, is the author of a long and 

 careful series of experiments, to ascertain the 

 heat developed in the combination of acids 

 and bases. The results prove that the heat 

 developed in the union is determined by the 

 base and not by the acid ; that, when a neutral 

 is converted into an acid salt by combining 

 with one or more atoms of acid, no change of 

 temperature occurs; and that, when a neutral 

 is converted into a basic salt by combining 

 with an additional proportion of base, the 

 combination is accompanied with the evolu- 

 tion of heat; and other laws ot minor interest 

 were also demonstrated. In the following 

 table, the author gives a summary of the more 

 important of his experiments, arranging the 

 acids in the order of their thermal action : 



He remarks that it is interesting to observe 

 how closely the results in the three vertical 

 columns agree relatively with one another. 

 The acids follow in the same order under each 

 base, and even the differences in the amount 

 of heat disengaged by the several acids in com- 

 bining with the different bases approximate 

 in many cases closely to one another. Thus 



