for the year 1880. xxi 



below its melting-point, while still retaining the fluid state. 

 This condition is called " supervision ;" and when at, last the 

 cooling alloy changes its state it does so suddenly with 

 evolution of heat of fluidity, and with a sudden glowing up 

 of remarkable brightness (the flashing). But the presence 

 o x f a few points or particles of solid iridium prevents super- 

 fusion, for the gold in contact with the iridium cannot 

 retain its fluid state below its melting-point ; each particle 

 of the iridium forms a nucleus from which the fluid metal 

 gradually solidifies and crystalises, with gradual and not 

 sudden evolution of heat. Thus from different sources we 

 gain a new insight into the conditions affecting a change of 

 state. 



A noteworthy instance of the practical application of 

 physiological and chemical science is presented by Fleuss's 

 method of diving. Under ordinary circumstances a diver 

 is supplied with a copious flow of air, forced by pumps 

 through a flexible tube into his helmet, but Mr. Fleuss dives 

 without air-pump or tube, and simply with a signal-line. 

 The method is this — First, he provides a store of oxygen 

 gas, compressed and stored in a special part of the helmet, 

 which is double-cased. This can be admitted to the interior 

 at will for breathing. Next, he arranges that ail air once 

 breathed shall be deprived of its poisonous carbonic acid, 

 and then be again returned to the helmet for respiration. 

 In this manner breathed and purified air can be breathed 

 again and again until the natural supply of oxygen becomes 

 partially exhausted, producing effects which Mr. Fleuss 

 instantly recognises, when he at once lets a small supply of 

 the compressed oxygen into his helmet to revivify the spent 

 air. For this purpose there is on the outside of the helmet 

 a tap which can be readily and instantaneously turned on 

 and off by the hand. The removal of the carbonic acid gas 

 from the expired air is effected by causing it to pass through 

 a tube containing a cellular mass of caustic alkali sufficient 

 to decarbonise the breath for four hours. 



