CHAP, x.] . COMBINED ENGINES. 505 



ammonia. It is known at the ordinary temperature of 15 water 

 dissolves 750 times its volume of ammoniacal gas, and that when it 

 is heated to 100 the gas dissolved entirely evaporates, and there is 

 no trace of it left in the water of solution. It is on this double pro- 

 perty that M. Frot has relied in the construction of his engine ; 

 experiments made by him on the tension of the gas at different 

 temperatures having proved that while at 100 C. it is 7| atmospheres, 

 at 120 C. it reaches 10 atmospheres. But in order to make the 

 employment of the elastic force of ammoniacal vapours economical 

 and practical two problems had to be solved ; first to condense 

 the vapour when it leaves the cylinder so as to obtain a sufficient 

 difference of pressure; and secondly to re-form the ammoniacal 

 solution, so as to use the same liquid as long as possible. 



This M. Frot has accomplished without essentially modifying the 

 arrangement of ordinary steam-engines. On leaving the cylinder, the 

 gases, after having exercised their force on the driving piston (at which 

 time they are composed of 1 part of steam to 5 parts of ammoniacal 

 gas), are led into a surface condenser formed of a triple series of tubes 

 round which a current of cold water is in constant circulation. In 

 order to render the condensation quicker, the nozzle of a pump throws 

 into the chamber which separates the two first ranges of tubes a non- 

 saturated solution of ammonia at a low temperature, which is itself 

 derived from the boiler. From the condenser the cooled and 

 partly dissolved gases are brought to a reservoir called the tubular 

 dissolver. There they become dissolved by contact with a non- 

 saturated solution of ammonia, and are brought thence by a feed- 

 ing pump into the boiler. During this last passage, the regenerated 

 solution goes across some twisted tubes plunged into the liquid, which, 

 as we have seen, serves for the injection. It here takes the heat that 

 the latter possesses, an arrangement doubly useful, since the feeding 

 solution enters warmer into the boiler, and the injected solution 

 reaches the condenser cooler. 



Since ammoniacal vapours attack copper, all the brass pieces of 

 ordinary machines have to be replaced by wrought-iron. The experi- 

 ments that have been made prove that the ammonia engine has 

 several advantages over ordinary steam-engines ; besides the economy 

 of fuel, which seems to be pretty considerable, and the rapidity with 

 which pressure is got up, we must include the almost total absence of 



