August 29, 1884.] 



SCIENCE 



181 



ment of the first law is partly responsible for the 

 little attention that is given to the second, for 

 the second law so far contradicts the usual statement 

 of the first as to assert that equivalents of heat and 

 work are not of equal value. While work can always 

 be converted into heat, heat can only be converted 

 into work under certain limitations. For every prac- 

 tical purpose, the work is worth the most; and, when 

 we speak of equivalents, we use the word in the same 

 sort of special sense as that in which chemists speak 

 of equivalents of gold and iron. The second law 

 teaches us that the real value of heat, as a source of 

 mechanical power, depends upon the temperature 

 of the body in which it resides : the hotter the body, 

 in relation to its surroundings, the more available 

 the heat. 



In order to see the relations which obtain between 

 the first and the second law of thermo-dynamics, it 

 is only necessary for us to glance at the theory of the 

 steam-engine. Not many years ago, calculations were 

 plentiful, demonstrating the inefficiency of the steam- 

 engine, on the basis of a comparison of the work 

 actually got out of the engine with the mechanical 

 equivalent of the heat supplied to the boiler. Such 

 calculations took into account only the first law of 

 thermo-dynamics, which deals with the equivalents 

 of heat and work, and had very little bearing upon 

 the practical question of efficiency, which requires 

 us to have regard, also, to the second law. Accord- 

 ing to that law, the fraction of the total energy which 

 can be converted into work, depends upon the rela- 

 tive temperatures of the boiler and condenser; and 

 it is therefore manifest, that, as the temperature of 

 the boiler cannot be raised indefinitely, it is impos- 

 sible to utilize all the energy which, according to the 

 first law of thermo-dynamics, is resident in the coal. 



On a sounder view of the matter, the efficiency of 

 the steam-engine is found to be so high that there is 

 no great margin remaining for improvement. The 

 higher initial temperature possible in the gas-engine 

 opens out much wider possibilities ; and many good 

 judges look forward to a time when the steam-engine 

 will have to give way to its younger rival. 



To return to the theoretical question, we may say, 

 with Sir W. Thomson, that, though energy cannot 

 be destroyed, it ever tends to be dissipated, or to pass 

 from more available to less available forms. No one 

 who has grasped this principle can fail to recognize 

 its immense importance in the system of the uni- 

 verse. Every change — chemical, thermal, or me- 

 chanical — which takes place, or can take place, in 

 nature, does so at the cost of a certain amount of 

 available energy. The foundations laid by Thomson 

 now bear an edifice of no mean proportions, thanks 

 to the labors of several physicists, among whom must 

 be especially mentioned Willard Gibbs, and Helm- 

 holtz. The former has elaborated a theory of the 

 equilibrium of heterogeneous substances, wide in its 

 principles, and, we cannot doubt, far-reaching in its 

 consequences. In a series of masterly papers, Helm- 

 holtz has developed the conception of free energy, 

 with very important applications to the theory of the 

 galvanic cell. He points out, that the mere tendency 



to solution bears, in some cases, no small proportion 

 to the affinities more usually reckoned chemical, and 

 contributes largely to the total electromotive force. 

 Also, in England, Dr. Alder Wright has published 

 some valuable experiments relating to the subject. 



From the further study of electrolysis, we may ex- 

 pect to gain improved views as to the nature of the 

 chemical reactions, and of the forces concerned in 

 bringing them about. Lord Kayleigh did not consider 

 himself qualified to speak on recent progress in gen- 

 eral chemistry; but if he might, without presump- 

 tion, venture a word of recommendation, it would 

 be in favor of a more minute study of the simpler 

 chemical phenomena. 



Under the head of scientific mechanics, it is prin- 

 cipally in relation to fluid motion that advances may 

 be looked for. The important and highly practical 

 work of the late Mr. Froude in relation to the pro- 

 pulsion of ships is, doubtless, known to most. Rec- 

 ognizing the fallacy of views widely held, as to the 

 nature of the resistance to be overcome, he showed, 

 that, in the case of fair-shaped bodies, we have to 

 deal almost entirely with resistance dependent upon 

 skin-friction; and, at high speeds, upon the genera- 

 tion of surface-waves, by which energy is carried off. 

 Although Professor Stokes, and other mathemati- 

 cians, had previously published calculations pointing 

 to the same conclusion, there can be no doubt that 

 the view generally entertained was very different. 

 Mr. Froude' s experiments have set the question at 

 rest in a manner satisfactory to those who had little 

 confidence in theoretical prevision. Although the 

 magnitude of skin-friction varies with the smooth- 

 ness of the surface, we have no reason to think that 

 it would disappear at any degree of smoothness con- 

 sistent with an ultimate molecular structure. That 

 it is connected with fluid viscosity is evident enough, 

 but the modus operandi is still obscure. 



Some important work bearing upon the subject has 

 recently been published by Prof. O. Reynolds, who 

 has investigated the flow of water in tubes as depend- 

 ent upon the velocity of motion, and upon the size of 

 the bore. The laws of motion in capillary tubes, dis- 

 covered experimentally by Poiseuille, are in com- 

 plete harmony with theory. The resistance varies as 

 the velocity, and depends in a direct maimer upon 

 the constant of viscosity. But, when we come to the 

 larger pipes and higher velocities with which en- 

 gineers usually have to deal, the theory which presup- 

 poses a regularly stratified motion evidently ceases 

 to be applicable, and the problem becomes essen- 

 tially identical with that of skin-friction in relation 

 to ship-propulsion. Professor Reynolds has traced 

 with much success the passage from the one state of 

 things to the other, and has proved the applicability, 

 under these complicated conditions, of the general 

 laws of dynamical similarity, as adapted to viscous 

 fluids by Professor Stokes. 



As also closely connected with the mechanics of 

 viscous fluids, an important series of experiments 

 upon the friction of oiled surfaces, recently executed 

 by Mr. Tower for the Institution of mechanical en- 

 gineers, must not be overlooked. When the lubrica- 



