ICE-MAKING MACHINES. 21 



lished which exists between the work expended by the compression- 

 pump and the cold produced in the shape of ice. 



The great differences which exist in the products of various ice- 

 machines are due solely to the practical considerations of which we 

 will now mention the chief. 



Ether machines have to exhaust very rarified vapours, which have a 

 tension of but a few centimetres of mercury. They consequently 

 require immense cylinders for the pump. And, moreover, the relative 

 vacuum of the refrigerator allows the exterior air to penetrate, by the 

 slightest opening, into the interior of the apparatus, and thus com- 

 pletely frustrates the work of the machine. It is necessary to grease 

 the piston of the pump, and this grease mingles thoroughly with the 

 ether ; this has the effect of diminishing the volatilizing power of this 

 liquid. Finally, after many volatilizations the chemical state of the 

 ether transforms itself into acid substances, which differ materially 

 from sulphuric ether. 



Accordingly, these machines do not work regularly in the factories 

 where they are used. 



The ammonia machine of Card avoids many of these defects, inas- 

 much as ammonia is much more volatile than ether, but there is a 

 greater difficulty in the use of the machine namely, the great pressure 

 which exists in the condenser and in the boiler. This pressure can 

 reach from eighteen to twenty atmospheres in hot countries, and thus 

 renders escape of gas inevitable and explosions much to be feared. 



The grease, also, saponifies itself with the ammonia, and quickly 

 transforms itself into soap. 



It can be perceived from these few words that the practical manu- 

 facture of cold for industrial purposes requires certain particular 

 conditions, which are not satisfied either by the ammonia or by the 

 ether machines. . 



It was to fulfil the various requirements of this question that I intro- 

 duced anhydrous sulphurous acid into the manufacture of ice. 



This liquid plainly satisfies all the conditions requisite for carrying 

 on the process of manufacture regularly. 



1. This liquid boils, under atmospheric pressure, at a temperature 



of 10 of cold 15 Fahrenheit. 



2. It never gives even in the tropics pressure greater than three 



