244 PROGRESS OF SCIENCE. 



transformation it may undergo. This energy exists either as 

 potential (or stored up) energy, which is hidden and invisible, 

 but present nevertheless ; or it exists as visible energy, when 

 it is performing work. It may change from one form into- 

 another, but its total amount cannot alter ; and this, whether 

 mechanical energy is changed into electrical energy, work 

 into heat, heat into work, and so forth. This is the LAW OF 

 CONSERVATION OF ENERGY, which Joule propounded and 

 which makes his name doubly famous. The law is thus 

 stated : " The total amount of energy in the imiverse is the 

 same at all times" By this great law Joule confirmed the 

 earlier doctrines of Leibnitz. He did more, for this law is a 

 eorroboration of Laplace and Lagrange's conclusions as 

 regards the stability of forces as exhibited in the invariability 

 of great axes of orbits, and in Laplace's Nebular theory. 

 Lastly, in its philosophical bearing, Joule's law substantiates 

 in the widest manner possible the paramount doctrine of 

 science the permanency of the government of the universe- 

 by unchangeable laws. It should be borne in mind that to 

 this result of Joule's, Sir William Thomson, Balfour Stewart,. 

 Clerk Maxwell, and others contributed largely. But it was- 

 Joule who demonstrated the law, one of the greatest 

 generalisations, if not the greatest, established since Newton- 

 It should also be stated that Thermodynamics, the subject 

 of Joule's earlier determination, have been developed since by 

 Rankine, Clausius, and the universal genius, Sir William 

 Thomson, who has probably exceeded all his rivals by the 

 vastness and value of the work he has done. 



1818 93. Tyndall investigated numerous sound phe- 

 nomena by a series of beautiful experiments, and notably 

 by means of his CHEMICAL HARMONICON an apparatus by 

 which the air in an open tube may be made to give 

 sounds by the introduction of a luminous jet of hydrogen 

 or gas, a phenomenon known under the name of singing 

 flames. * The note depends on the size of the flame and 

 the length of the tube. The apparatus was probably 

 suggested by the SYREN of Cagniard Latour, which is 

 used to measure the number of vibrations of a body in 

 a given time: "with the same velocity the Syren gives 



