VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 259 



which she quits them, besides a few occasional remarks. It is disposed in a much 

 coarser manner than the Benares patra, as each page contains as many days as it 

 will hold, so that the month seldom begins at the beginning of a page. It con- 

 tains no preface, and no explanation of the columns. The days of the week, are 

 not denoted by the first syllables of the name, but only by a number, expressing 

 their order in the week, which caused some trouble in finding what day was 

 meant by these numbers ; but, by a variety of circumstances, Mr. C. thinks it 

 certain that the number 1 must denote Sunday. 



XXL On Evaporation. By John Andrew de Luc, Esq. F. R. S. p. 400. 



In Mr. D.'s last papers on hygrometry, he considered moisture in the air as a 

 modification of a particular fluid, produced by the evaporation of water, com- 

 posed of water and fire, mixed with the air, but independent of it. However 

 there was a more common theory of that phenomenon, in which evaporation was 

 attributed to a dissolution of water by air : but as an inquiry into the cause of 

 evaporation belongs more to hygrology than to hygrometry, he made then no re- 

 mark on that subject ; having in view some experiments which were to ascertain 

 a particular point fundamental to it. Since that time he has made those experi- 

 ments, which are the object of this paper; but before relating them, it is neces- 

 sary to explain how they connect hygrometry with hygrology ; which will be by 

 stating the principles of those two branches of experimental philosophy according 

 to his system. 



From the time Mr. D. fixed his attention on evaporation, and its various con- 

 sequences, he was led to think, that the kind of dissolution of water, observed in 

 those phenomena, was operated by fire, without any interference of air : and 

 among other reasons for that opinion, the most decisive was, that every liquid 

 cools when it evaporates ; for he considered that circumstance as a proof, that the 

 portion of the liquid which then disappears, is carried away by a quantity of fire 

 proceeding from the liquid itself. Mr. D. acknowledges himself indebted to Mr. 

 Watt, for an immediate proof of his fundamental opinion, resulting from an ex- 

 periment, which he repeated in his presence, and which demonstrates, that in 

 the common evaporation of water in open air, the quantity of heat lost by the 

 mass, bears to the quantity of water carried away, a proportion still greater than 

 that which is found in the steam produced by boiling water. Therefore he thinks 

 there is no reason to doubt, that steam is formed in the first, as in the last of 

 those cases. 



On the laws of hygrology, Mr. D. observes, 1st. that whenever water is in a 

 state of evaporation, an expansible fluid, called steam, composed of water and 

 fire, is produced. 2. That as long as steam exists, it has a power of pressure as 

 air itself; but it does not belong to the class of permanent fluids, for it may be 



L L 2 



