HYGROMETRY. 



589 



by 75. It appears by various experiments which have 

 been made with the hygrometer of Saussure, that when 

 the receiver in which it is inclosed contains air com- 

 H pletely saturated with moisture, whatever be its tempe- 

 t of rature and density the index always arrives at a fixed 

 ku- point in the scale, namely, that of extreme humidity ; 

 " and hence it has been justly inferred, that the indica- 

 tions of this instrument, as well as the degrees marked 

 by hygrometers in general, are to be regarded as having 

 reference to the relative, and not to the absolute 

 quantity of moisture contained in the medium to which 

 they are exposed ; as expressing, in short, the relation 

 of the quantity of moisture actually existing in that me- 

 dium, to the whole quantity which could exist in it at 

 the particular temperature when the observation is 

 made. This view of the subject will receive consider- 

 able elucidation, by considering the hygroscopic sub- 

 stance as exerting an affinity for moisture ; and, like 

 other desiccmttvem, reducing the vapour to which it is 

 exposed to the liquid or perhaps the solid state, until its 

 affinity is counterbalanced by the increasing tendency 

 of the moisture to maintain itself in the vaporous con- 

 dition. In all case* of complete saturation, the smallest 

 attraction for moisture must be sufficient to precipitate 

 the vapour ; and since the quantity of water absorbed 

 by the hair is so very minute that it can produce no 

 sensible change in the density of the vapour, it is easy 

 to perceive, that when the medium in which the hygro- 

 meter is placed is completely charged with moisture,' 

 the hair must continue to abstract water, until its affi- 

 nity for that fluid terminates in saturation. Whatever, 

 therefore, be the temperature or the elasticity of the 

 vapour, provided it be at its maximum quantity for that 

 temperature, the hair must absorb the same portion of 

 and consequently reach the same degree of 



i.'. : . 

 BSM be> 

 . - 

 ret drynrt 

 d bumi- 

 . 



The result however must be very different, when 

 the instrument i inclosed in a medium which contains 

 vapour possessing a tension less than that of the maxi- 

 mum for the temperature. In this case, the hair will 

 only continue to absorb moisture till its affinity is coun- 

 terbalanced by a force proportional to/j /', or the 

 difference between the whole elastic force of vapour cor- 

 responding to complete saturation at the given tempera- 

 ture, and the elastic force of vapour at the same tempera- 

 ture corresponding to the actual quantity of moisture in 

 the medium. The point at which an equipoise is esta- 

 blished between these opposing forces, will depend upon 

 the law by which the affinity of the hair for moisture is 

 modified by temperature, and the quantity of water al- 



ready absorbed. Saussure attempted to discover this law 

 by experiment ; but, though his researches were con- 

 ducted with very great care, the results he has given 

 can scarcely be regarded sufficiently accurate to autho- 

 rise our application of them to circumstances greatly 

 different from those in which they were obtained. The 

 problem, however, has been solved by Gay Lussac in a 

 manner so general and satisfactory, as to raise this 

 branch of hygrometry from an empirical collection of 

 facts to a subject of the most precise and rigid ana- 

 lysis.* 



77. Having procured an hygrometer, on the accuracy 

 of which he could sufficiently rely, Gay Lussac inclosed 

 it in a receiver with a quantity of water, or a solution 

 of some salt of a known specific gravity, and observed 

 the degree marked by the instrument, corresponding to 

 the saturation of the space with the vapour which was 

 formed. He thus determined the degrees of the hygro- 

 meter answering to the observed tension of the liquid ; 

 and by making experiments, at the same temperature, 

 for different tensions, between extreme dryness and 

 complete saturation with the vapour of water, he pro- 

 cured as many terras of the correspondence as were 

 deemed necessary. He obtained in this manner, at the 

 temperature of 10 centigrade (50 Fahrenheit), the re. 

 suits laid down in the following Table, in which the 

 tensions are expressed in decimal parts of the tension 

 of water, the latter being denoted by unity : 



Hygrome. 

 try. 



Research e< 

 of Gay Lus- 

 sac to deter- 

 mine a for- 

 mula far 

 Saussure's 

 hygrome- 

 ter. 



78. To form a general idea of the nature of these re- 

 sults, it will be found very convenient to represent by Pt ATB 

 a diagram the tensions reckoned from a given point A cccxxv j 

 low the line A B for the abscissa-, and the correspond- Kj 3 

 ing degrees of the hygrometer as rectangular ordinates 

 of the hygrometrical curve. If the former be repre- 

 sented by i, and the Utter by y, then at the origin A 

 of the line of abscissae, x = 0, and y = 0; while at the 

 opposite extremity ( AB) * = 100, and ( BC) y = 100, 

 because 100 degrees of the instrument correspond to 

 the tension denoted by 100. 



By laying down from a scale of equal parts, the 

 abtcuHf and ordinates belonging to the intermediate 

 results, Gay Lussac found that the line connecting the 

 extremities of the ordinates was an hyperbola, concave 

 towards the line of the abscissae, and having its axis 

 BV inclined to the same line at an angle of 45. The 

 axis cuts the line of the abscissae at the point where * is 



nt in tke text of UM method of tartsUfttion tsaplored by Gay I.unse, if ratrsctsd from the admirable work of M- Blot, 

 d, entitled, TniU 4* fkytifn S*ftnmtnlfU ft MtHitmalii*e, torn. ii. p. 199. 



