Condensation of Air and Steam upon Cold Surfaces, 67 



Silver and copper with deoxygenized copper nitrate 4 



Ditto „ oxygenized ditto 8 to 11 . 



Copper and zinc with chloride of zinc 62 



Ditto „ water 68 



Ditto „ Daniell's cell 83 



Chloride of zinc is preferred to the sulphate, as it offers less 

 internal resistance, and a solution of 20 per cent, is recommended 

 as about the best conductor*. A single cell of this description is 

 capable of decomposing dilute sulphuric or hydrochloric acid when 

 copper electrodes are employed. The two metals might be arranged 

 as in a Daniell's battery ; the zinc would of course require no amal- 

 gamation ; and the whole might be left for weeks or months without 

 any attention. The oxide of zinc produced generally falls to the 

 bottom of the vessel, and may be separated whenever it is thought 

 desirable. 



The power is thus obtained at a minimum of expense ; for the 

 oxygen which combines with the zinc costs nothing. Such a battery 

 would appear to be specially adapted to cases where the galvanic 

 current has to be frequently broken, as in telegraphy ; for at each 

 period of rest it renews its strength by the absorption or diffusion 

 of more oxygen from the air. 



May 1, 1873. — William Spottiswoode, M.A., Treasurer and 



Vice-President, in the Chair. 

 The following communication was read : — 



" On the Condensation of a Mixture of Air and Steam upon 

 Cold Surfaces." By Osborne Beynolds, M.A., [Fellow of Queen's 

 College, Cambridge. 



1. The object of this investigation is to ascertain how far the 

 presence of a small quantity of air affects the power of a cold sur- 

 face to condense steam. A priori it seemed probable that it might 

 retard condensation very much ; for when pure steam comes up to a 

 cold surface and is condensed, it leaves an empty space which is im- 

 mediately filled with fresh steam ; so that the passage of the steam 

 up to the cold surface is unobstructed, and if the surface could 

 carry off the heat fast enough, then the rate of condensation would 

 be unlimited. If, however, the steam is mixed with air, then, as 

 the mixture comes into contact with the cold surface, the steam will 

 be condensed and the air will be left between the fresh steam and 

 the cold surface ; so that after condensation has commenced that 

 surface will be protected by a stratum of air, and fresh steam will 

 have either to displace this or pass through it before it in turn can 

 be condensed. 



2. This question, besides its philosophical interest, has important 

 practical bearings on the steam-engine. 



First. If the quantity of air mixed with the steam affects the 



rate at which it condenses, then the ratio which the pressure of air 



bears to the pressure of steam in a condenser will materially affect 



* On the authority of Mr. Herbert MacLeod. 



F2 



