74 



POPULAR SCIENCE NEWS. 



[May, 1889. 



ever, are those of the pectoral fins situated 

 farthest forward on the body. We find them 

 here dimhiished in size, and terminated by an 

 enhirged, fleshy mass, which shows no trace 

 of being webbed like an ordinary fin, and, 

 in fact, transformed into little feet, their 

 original adaption to swimming purposes hav- 

 ing entirely disappeared. The ventral or 

 posterior fins, are even more modified, and 

 resemble more closely the hind legs of a frog 

 than the swimming members of an ordinary 

 fish. The anal fin also, as shown in the 

 view of the fish from below, instead of project- 

 ing outwardly, is flattened horizontally against 

 the body of the ani- 

 mal, and is thus a 

 further aid to loco- 

 motion. 



It is interesting to 

 compare a fish like 

 the Malthe which has, 

 as it were, been par- 

 tially changed into a 

 walking animal, with 

 the flying fish, whose ,, 

 fins have been modi- 

 fied in an entirely dif- 

 ferent way, towards 

 the condition of 

 wings. The closer 

 the difl'erent forms of 

 animal and vegetable 

 life are studied, the 

 more clearly we per- 

 ceive the relations be- 

 tween them, and the 

 more numerous and 

 convincing become 

 the proofs of the ori- 

 gin of the different forms by the processes of 

 modification, variation, and natural selection. 



oxidation in which a perceptible amoimt of 

 heat is developed. If concentrated sulphuric 

 acid be mixed with water, the mixture be- 

 comes extremely hot, and may even rise 

 above the boiling point of water. The best 

 proportions are four parts of acid to one part 

 of water, but the experiment is not entirely 

 safe, and should only be performed with 

 small quantities. The acid should always be 

 poured into the water, and not the water into 

 the acid. A mixture of common alcohol and 

 water shows the same phenomena in a lesser 

 degree. 



If four parts of sulphuric acid be added to 



HEAT AND COLD IN CHEMICAL 

 REACTIONS. 



In nearly every chemical reaction there is a 

 disturbance of the thermic equilibrium : that 

 is to say, heat is either developed or absorbed. 

 This is very evident in those reactions like the 

 combination of hydrogen or carbon with oxy- 

 gen, which we term combustion, but it is none 

 the less so in the slower and less energetic re- 

 actions. If we burn a steel wire in oxygen 

 gas, the brilliant sparks show that a large 

 amount of heat is developed, but, leave tlje 

 same wire exposed to the action of air and 

 water until it is all converted into rust or 

 oxide, and exactly as much heat will be pro- 

 duced as in the rapid oxidation in the jar of 

 oxygen, only in the latter case it is produced 

 so slowly that it is not perceptible to our 

 senses. So if a stick of wood decays and 

 rots on the ground in the forest, just as much 

 heat is developed as if it had been burnt in a 

 fireplace. 



There are other reactions besides that of 



one part of snow, the latter is immediately 

 melted and the temperature rises, but if one 

 part of acid be mixed with four parts of 

 snow, the result is reversed and there is a 

 lowering of the temperature. In the first 

 case, the heat produced by the union with 

 the acid is greater than that absorbed by the 

 melting of the ice, but in the latter case the 

 conditions are reversed. 



In the class of reactions described above, 

 heat is developed or set free, but there are 

 other classes of reactions, in which heat is 

 absorbed, or cold is produced. One of the 

 most familiar of these is that of a mixture of 

 salt and ice, in which the ice, in the act of 

 being liquefied by the salt, absorbs so much 

 heat that a temperature approximating to zero 

 is produced. But the presence of ice is 

 unnecessary, as nearly every salt, while dis- 

 solving in water, absorbs a certain amount of 

 heat, and this is pre-eminently the case with 

 nitrate of ammonia, which, when mixed with 

 a certain proportion of water, reduces the 

 temperature of the mass below the freezing 

 point ; and numerous other instances of the 

 development of cold in connection with 



chemical and physical change might be given. 

 In general, it may be said that, in processes 

 where chemical atoms combine with each 

 other, heat is developed, and, conversely, in 

 processes of decomposition, where the atoms 

 are torn apart, heat is absorbed and cold is 

 produced. Everv pound of iron, which, in 

 the process of smelting, is reduced from its 

 ore and separated from the oxygen with 

 which it is combined, absorbs in the process a 

 certain amoimt of heat, and, although the 

 interior of a blast furnace is a strange place 

 to look for the development of cold, yet it is 

 true that a certain amount of heat is there 

 absorbed by the iron, 

 not to be given out 

 ;ain until the metal 

 is burned or oxidized. 



There are, how- 

 ler, certain excep- 

 tions to the rule given 

 above. When per- 

 oxide of hydrogen, 

 Siy iodide of nitrogen, 

 or even nitro-glycer- 

 ine, are decomposed, 

 -at, and not cold, 

 produced. These 

 exceptions arc more 

 ijjparent than real, 

 or in these decom- 

 M positions, the atoms 

 are really brought 

 nearer together, just 

 as in chemical com- 

 bination, and the rule 

 still holds. So, con- 

 versely, in the forma- 

 tion of the above sub- 

 stances by the combination of their elements, 

 cold, and not heat, is produced. 



It is a point of the utmost importance that 

 the amount of heat developed or absorbed in 

 any given reaction is always the same, and 

 governed by mathematical laws. A pound 

 of carbon, in uniting with oxygen to form 

 carbonic dioxide, produces a perfectly definite 

 quantity of heat, wliich is always the same. 

 Even if it does not unite directly, but passes 

 through the intermediate stage of carbonic 

 oxide, the total amount of heat produced by. 

 the two reactions is still exactly the same. 

 A pound of iron is burnt or rusted, and just 

 as much heat is set free as was absorbed by it 

 from the fires of the blast furnace in which it 

 was reduced. If, instead of oxygen, it unites 

 with sulphur, a different, but still absolutely 

 definite and unchangeable quantity of heat is 

 developed, and so whatever reaction we may 

 take, the simplest or the most complex, the 

 amount of heat involved is strictly dependent 

 upon mathematical laws, and is a ftxed and 

 invariable quantity. 



The same laws which govern the solution 

 of a limip of sugar in a cup of coffee, apply 



