138 



NATURE 



{Dec. 21, 1871 



mechanical force expended by the atoms in falling towards 

 one another, which determines the intensity of the current, 

 and, consequently, the quantity of heat evolved ; so that 

 we have a simple hypothesis by which we may explain 

 why heat is evolved so freely in the combination of gases, 

 and by which indeed we may account ' latent heat ' as a 

 mechanical power prepared for action as a watch-spring 

 is when wound up. Suppose, for the sake of illustration, 

 that 8 lbs. of oxygen and I lb. of hydrogen were pre- 

 sented to one another in the gaseous state, and then 

 exploded ; the heat evolved would be about 1° F. in 

 60,000 lbs. of water, indicating a mechanical force ex- 

 pended in the combination equal to a weight of about 

 50,000,000 lbs. raised to the height of one foot. Now 

 if the oxygen and hydrogen could be presented to each 

 other in a liquid state, the heat of combination would be 

 less than before, because the atoms in combining would 

 fall through less space." No words of mine are needed to 

 point out the commanding grasp of molecular physics, in 

 their relation to the mechanical theory of heat, implied by 

 this statement. 



Perfectly assured of the importance of the principle 

 which his experiments aimed at establishing, Mr. Joule 

 did not rest content with results presenting such discre- 

 pancies as those above referred to. He resorted in 1844 

 to entirely new methods, and made elaborate experiments 

 on the thermal changes produced in air during its expan- 

 sion : firstly, against a pressure, and therefore performing 

 work ; secondly, against no pressure, and therefore per- 

 forming no work. He thus established anew the relation 

 between the heat consumed and the work done. From 

 five different series of experiments he deduced five different 

 mechanical equivalents ; the agreement between them 

 being far greater than that attained in his first experi- 

 ments. The mean of them was 802 foot-pounds. From 

 experiments with water agitated by a paddle-wheel, he 

 deduced, in 1845, an equivalent of 890 foot-pounds. In 

 1 847 he again operated upon water and sperm-oil, agitated 

 them by a paddle-wheel, determined their elevation of 

 temperature, and the mechanical power which produced 

 it. From the one he derived an equivalent of 781 '5 foot- 

 pounds ; from the other an equivalent of 782'i foot- 

 pounds. The mean of these two very close determina- 

 tions is 78r8 foot-pounds. 



At this time the labours of the previous ten years had 

 made Mr. Joule completely master of the conditions 

 essential to accuracy and success. Bringing his ripened 

 experience to bear upon the subject, he executed in 1849 

 a series of 40 experiments on the friction of water, 50 

 experiments on the friction of mercury, and 20 experi- 

 ments on the friction of plates of cast-iron. He deduced 

 from these experiments our present mechanical equivalent 

 of heat, justly recognised all over the world as " Joule's 

 equivalent." 



There are labours so great and so pregnant in conse- 

 quences, that they are most highly praised when they are 

 most simply stated. Such are the labours of Mr. Joule. 

 They constitute the experimental foundation of a principle 

 of incalculable moment, not only to the practice, but still 

 more to the philosophy of Science. Since the days of 

 Newton, nothing more important than the theory of which 

 Mr. Joule is the experimental demonstrator has been 

 enunciated. 



1 have omitted all reference to the numerous minor papers 

 with which Mr. Joule has enriched scientific literature. Nor 

 have 1 alluded to the important investigations which he 

 has conducted jointly with Sir William Thomson. But 

 sufficient, 1 think, has been here said to show that, in 

 conferring upon Mr. Joule the highest honour of the 

 Royal Society, the Council paid to genius not only a well- 

 won tribute, but one which had been fairly earned twenty 

 years previously.* 



Comparing this brief history with that of the Copley 

 Medalist of 187 1, the differentiating influence of" environ- 

 ment " on two minds of similar natural cast and endow- 

 ment comes up in an instructive manner. Withdrawn 

 from mechanical appliances, Mayer fell back upon reflec- 

 tion, selecting with marvellous sagacity from existing 

 physical data the single result on which could be founded 

 a calculation of the mechanical equivalent of heat. In 

 the midst of mechanical appliances. Joule resorted to 

 experiment, and laid the broad and firm foundation 

 which has secured for the mechanical theory the ac- 

 ceptance it now enjoys. A great portion of Joule's time 

 was occupied in actual manipulation ; freed from this, 

 Mayer had time to follow the theory into its most ab- 

 struse and impressive applications. With their places 

 reversed, however. Joule might have become Mayer, and 

 Mayer might have become Joule. 



John Tyndall 



THE BROWN INSTITUTION 



IN 1852 a large sum of money was bequeathed by the 

 late Mr. Thomas Brown to the University of London 

 for the purpose of " founding and upholding " an Insti- 

 tution for "investigating, studying, and if possible en- 

 deavouring to cure " the diseases and injuries of animals 

 useful to man. The sum was to be allowed to accumulate 

 for a limited period, at the end of which the principal and 

 interest w-ere to be applied in the manner directed. And 

 it was provided that in case the University should fail to 

 carry out the trust imposed upon it within nineteen years 

 after the testator's death, the whole sum with the accumu- 

 lations should be transferred to the University of Dublin, 

 to be applied for the endowment of certain philological 

 professorships. The will contains various directions for 

 the administration of the proposed Institution. The most 

 important are those which relate to the appointment of a 

 Committee of Management and of a Professor. The 

 committee must be appointed by the governing body of 

 the University, and must consist either of members of the 

 Senate or of other persons, members of the medical pro- 

 fession. As regards the qualifications of the professor 

 nothing is said. He must be appointed by the University, 

 must give a course of lectures annually, and must have a 

 residence adjacent to the Institution. 



The nineteen years have now almost expired. In pur- 

 suance of the testator's directions, the " Brown Institution " 

 has just been opened. Last summer a large plot of free- 

 hold land was acquired by the University in the Wands- 

 worth Road, close to the goods station of the South- 



* Had I found it in time, this notice should have preceded th3t of the 

 Copley Medalist of 1871. 



