280 Royal Society. 



of indigo are however frequently found in urines possessing charac- 

 ters the very reverse, that is, in such as are high-coloured, acid, and 

 of high specific gravity ; but, as a rule, in these urines the blue pig- 

 ment is usually absent. 



5thly. That as coloured indigo does not occur in healthy urine, 

 and since where the amount of this is at all considerable it is ac- 

 companied with strongly-marked symptoms of deranged health, the 

 formation of blue indigo in urine must be regarded as a strictly 

 pathological phenomenon, apparently associated rather with some 

 general morbid condition, than essentially with disease of any one 

 organ ; although there is reason for believing that the blue deposit 

 is met with very frequently in Bright's disease, and in affections of 

 the organs of respiration, it should however be remarked that none 

 of the worst cases of indigo in the urine which the author met with 

 were cases of Bright's disease. 



The paper is illustrated by drawings, and a specimen of the indigo, 

 as deposited from urine, was exhibited. 



4. " On the Thermal Effects of Elastic Fluids." By Professor 

 William Thomson, F.R.S., and J. P. Joule, Esq., F.R.S. 



The authors had already proved by experiments conducted on a 

 small scale, that when dry atmospheric air, exposed to pressure, is 

 made to percolate a plug of non-conducting porous material, a de- 

 pression of temperature takes place increasing in some proportion 

 with the pressure of the air in the receiver. The numerous sources 

 of error which were to be apprehended in experiments of this kind 

 conducted on a small scale, induced the authors to apply for the 

 means of executing them on a larger scale ; and the present paper 

 contains the introductory part of their researches with apparatus 

 furnished by the Royal Society, comprising a force pump worked by 

 a steam-engine and capable of propelling 250 cubic inches of air per 

 second, and a series of tubes by which the elastic fluid is conveyed 

 through a bath of water, by which its temperature is regulated, a 

 flange at the terminal permitting the attachment of any nozle which 

 is desired. 



Preliminary experiments were made in order to illustrate the ther- 

 mal phenomena which result from the rush of air through a single 

 aperture. Two effects were anticipated, one of heat arising from 

 the vis viva of air in rapid motion, the other of cold arising from 

 dilatation of the gas and the consequent conversion of heat into me- 

 chanical effect. The latter was exhibited by placing the bulb of a 

 very small thermometer close to a small orifice through which dry 

 atmospheric air, confined under a pressure of 8 atmospheres, was 

 permitted to escape. In this case the thermometer was depressed 

 1 3° Cent, below the temperature of the bath. The former effect was 

 exhibited by causing the stream of air as it issued from the orifice 

 to pass in a very narrow stream between the bulb of the thermo- 

 meter and a piece of gutta percha tube in which the latter was en- 

 closed. In this experiment, with a pressure of 8 atmospheres, an 

 elevation of temperature equal to 23° Cent, was observed. The same 



