466 Royal Society. 



considers that, connected as the form of the shell is with the cir- 

 cumstances of the animal's growth and the manner of its existence, 

 this number, or the angle of the particular spiral, determinable as 

 it is in each case by actual measurement, may be available for the 

 purposes of classification, and may suggest relations by which, 

 eventually, they may become linked with characteristic forms, and 

 modes of molluscous existence. 



The concluding portion of the paper contains a mathematical dis- 

 cussion of certain geometrical and mechanical elements of a con- 

 choidal surface. These are, the extent of the surface itself; the vo- 

 lume contained by it ; the centre of gravity of the surface, and also 

 of the volume, in each case, when the generating figure revolves 

 about a fixed axis without any other motion, and also when it has, 

 besides this, a motion of translation an the direction of that axis ; 

 and, lastly, the angle of the spiral. The author states that his ob- 

 ject in this inquiry is the application of these elements to a discussion 

 of the hydraulic theory of shells. The constant angle of the spiral, 

 which each particular species afi^ects, being connected by a necessary 

 relation with the economy of the material of the habitation of each, 

 with its stability, and the condition of its buoyancy, it is therefore 

 necessary to determine the value of this angle. 



" On the relative attractions of Sulphuric Acid for water, under 

 particular circumstances ; with suggestions of means of improving 

 the ordinary process of manufacturing Sulphuric Acid." By Henry 

 H. Watson, Esq., Corresponding Member of the Manchester Philoso- 

 phical Society. Communicated by John Dalton, D.C.L., F.R.S., &c. 



The object of the inquiry detailed in the present paper is to de- 

 termine at what degree of concentration the affinity of sulphuric 

 acid for aqueous vapour is equal to that of anhydrous space for the 

 same vapour at given temperatures. It has long been known that 

 concentrated sulphuric acid abstracts moisture from the atmosphere, 

 but the amount and the rate of this absorption have never been as- 

 certained with accuracy ; and consequently, in applying this acid to 

 purposes of exsiccation, the experimenter has often been at a loss to 

 know whether the acid was sufficiently strong to render the space 

 in which it was confined perfectly anhydrous. By placing portions 

 of the acid, previously weighed, and diluted with known quantities 

 of water, under the receiver of an air-pump, with equal portions of 

 concentrated acid, of the specific gravity 1-8428, in similar dishes, 

 the author ascertained that the dilute acid could be concentrated to 

 the specific gravity 1'814, at a temperature varying from 65° to 

 57° : whence he concludes that acid of such strength is capable 

 of drying a vacuum when the temperature does not exceed 5 7°. By 

 making similar experiments in air, the author compared together the 

 weights lost by ten grains of dilute sulphuric acid of the specific 

 gravity 1-135, at three diflferent periods of the day for six days, 

 taking note of the dew-point and the temperature ; and infers that 

 when the afl!inity of space for vapour, or the evaporating force, is 

 equal to 0-15 of an inch of mercury, it is just able to balance the 

 affinity for water of sulphuric acid of the specific gravity 1-249. 



