438 



NATURE 



[March io, 1892 



entering into the minor details, will find in this little book 

 a most useful guide. The author has dealt with the sub- 

 ject rather curtly, but nevertheless in this space the 

 reader will find descriptions of various lanterns for different 

 methods of projection ; hints on the most suitable posi- 

 tions in which screens should be placed to be best viewed 

 by audiences ; the best kinds . of burners for the lamps, 

 both oil and oxy-hydrogen, and the different adjustments 

 for producing good results. Many other useful hints are 

 given, accompanied by several woodcuts. W. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of ^ATVK^. 

 No notice is taken of anonymous communications. ] 



Heat-Engines and Saline Solutions. 



Mr. Macfarlane Gray (p, 414) appears to call in question 

 my assertion that in a vapour-engine a saline solution may take 

 the place of a simple liquid when it is desired to replace water 

 by a substance of less volatility, and that the advantage which 

 Carnot proved to attend a high temperature can thus be attained 

 without encountering an unduly high pressure. He contends 

 that " the saline mixture is not the working substance. Carnot's 

 law refers to the working substance only, and not to anything 

 left in the boiler." 



Perhaps the simplest way of meeting this objection is to point 

 out that Maxwell's exposition of Carnot's engine ("Theory of 

 Heat," chapter viii.) applies without the change of a single word, 

 whether the substance in the cylinder be water, mercury, or an 

 aqueous solution of chloride of calcium. In each case there is a 

 definite relation between pressure and temperature ; and (so far 

 as the substance is concerned), all that is necessary for the re- 

 versible operation of the engine is that the various parts of the 

 working substance should be in equilibrium with one another 

 throughout. 



Let us compare the behaviour of water in Carnot's engine 

 before and after the addition of chloride of calcium, supposing 

 that the maximum and minimum pressures are the same in the 

 two cases. The only effect of the addition is to raise both the 

 superior and the inferior temperatures. The heat rejected at 

 the inferior temperature may still be available for the convenient 

 operation of an engine working with pure water. At the upper 

 limit, all the heat is received at the highest point of tempera- 

 ture — a state of things strongly contrasted with that which 

 obtains when vapour rising from pure water is afterwards super- 

 heated. Rayleigh. 



Temperatures in Model Boiler working tip to lo pounds pressure- 



Superheated Steam. 



Lord Rayleigh touches on a most important question 

 (February i8, p. 375), which merits the attention of all inter- 

 ested in the economy of prime movers. Few have troubled 

 themselves with determinations of temperatures and pressures 

 within a steam generator. Ebullition means work, and the per- 

 formance of work involves cooling ; hence the temperature of 

 steam in the steam space of any boiler is lower by several 

 degrees than the temperature of the steaming water. I have 

 failed to find any record of this important truth, and shall be 

 glad to know if my observations have been anticipated. 



Prof. Cotterill, in his work on the steam-engine (p. 33), re- 

 ferring to the process of formation of steam under rising pressure 

 in a closed vessel, says: — "The mixture of steam and water 

 must be supposed so treated that the temperature is sensibly 

 uniform. If the experiment were tried without proper precau- 

 tions, the steam would probably be found to be of higher tem- 

 perature than the water — that is, it would be superheated." So 

 far as my observations go, this is impossible, and the steam is 

 never superheated by compression in a closed vessel, in contact 

 with water. 



In a small experimental boiler the records of temperature 

 indicated as follows : — 



NO. II 67, VOL. 45] 



To avoid supersaturation of the steam it must be separated as- 

 promptly as possible from the water, which it projects, more or 

 less, into the steam space. It is this which renders it so im- 

 portant in practice to secure the most active circulation. Pro- 

 vision for this, whereby the water falls, whilst the steam rises, 

 can be made. 



Uniformity of temperature of the boiler contents is of the 

 utmost importance ; and I was recently told by an able engineer, 

 connected with the Midland Railway, that the unequal expan- 

 sion of the boiler plates in locomotives on getting up steam was 

 not only disastrous in its consequences, but impossible of pre- 

 vention. Pursuing thermometric experiments, I found this not 

 to be the case, and on a first trial of suitable apparatus, I 

 obtained the following result : — 



Model Locomotive Boiler, showing Hottest Water at the Bottom 

 under 212° {October 24, 1 891). 



Lord Rayleigh's suggestion to use liquids of higher boiling- 

 point than water, such as saline solutions, to get hotter steam 

 whereby to raise the upper limit of temperature in a steam- 

 engine, is not feasible. Increased elasticity of steam or increased 

 tension was long since shown by John Sharpe ("Annals of 

 Phdosophy," vol. i. p. 459, 1813) to be due to a corresponding 

 increase in its density. He pointed out that at 212° the density 

 of steam was 150 times greater than at 33°, and at 252° it was 

 twice as great as at 212°. Increasing the density of the liquid 

 does not help us, but liquids of lower boiling-point yield vapours 

 of higher density than steam at equivalent temperatures. An- 

 hydrous ammonia vapour exerts a pressure of 4 atmospheres at 

 32°, and its density is about o-2, whereas at 120° F. the pressure 

 is in round figures 285 pounds on the square inch, and its density 

 0-850. 



Properties of Saturated Steam as compared with Saturated 

 (^Anhydrous) Ammonia Vapoicr. 



