394 PLATEAU ON SOAP-BUBBLES. 



has ever since been blind. But in spite of this great loss he lias continued 

 for many yean to carry on experiments such as those described in this book, 

 ..n the forma of liquid masses and films, which he himself can never either see 

 >r handle, but from which he gathers the materials of science as they are 

 furnished to him by the hands, eyes, and minds of devoted friends. 



So perfect has been the co-operation with which these experiments have 

 been carried out, that there is hardly a single expression in the book to 

 indicate that the measures which he took and the colours with which he was 

 charmed were observed by him, not in the ordinary way, but through the 

 mediation of other persons. 



Which, now, is the more poetical idea the Etruscan boy blowing bubl.Ks 

 for himself, or the blind man of science teaching his friends how to blow tlirm, 

 and making out by a tedious process of question and answer the conditions 

 >f the forms and tints which he can never see ? 



But we must now attempt to follow our author as he passes from phe- 

 nomena to ideas, from experiment to theory. 



The surface which forms the boundary between a liquid and its vapour is 

 the seat of phenomena on the careful study of which depends much of our 

 future progress in the knowledge of the constitution of bodies. To take the 

 simplest case, that of a liquid, say water, placed in a vessel which it does not 

 fill, but which contains nothing else. The water lies at the bottom of the 

 vessel, and the upper part, originally empty, becomes rapidly filled with the 

 vapour of water. The temperature and the pressure the quantities on which 

 the thermal and statical relations of any body to external bodies depend are 

 the same for the water and its vapour, but the energy of a milligramme of 

 the vapour greatly exceeds that of a milligramme of the water. Hence the energy 

 of a milligramme of water-substance is much greater when it happens to be in 

 the upper part of the vessel in the state of vapour, than when it hapj>en8 

 to be in the lower part of the vessel in the state of water. 



Now we find by experiment that there is no difference between the phe- 

 nomena in one part of the liquid and those in another part except in a region 

 close to the surface and not more than a thousandth or perhaps a millionth 

 of a millimetre thick. In the vapour also, everything is the same, except 

 perhaps in a very thin stratum close to the surface. The change in the value 

 of the energy takes place in the very narrow region between water and vapour. 

 Hence the energy of a milligramme of water is the same all through the mass 



