246 SIXTH REPORT — 1836. 



therefore be pronounced to be satisfactory. As the formula was 

 obtained without taking into account the effect of the develop- 

 ment of heat, we may infer from the small difference between the 

 above results, that this effect, if not wholly insensible, is of very 

 small amount. M. Colladon remarks respecting the nature of 

 the sound transmitted through the water, that when caused by 

 the striking of a bell, it was heard as a sharp and dry sound, 

 resembling the striking of two knife-blades against each other. 

 This fact seems to prove with respect to liquids, what is also 

 most probably true of solids, that the relation between their 

 density and pressure is such as not to allow the condensations 

 arising from any disturbing cause to be transmitted to great di- 

 stances exactly in the order and proportion in which they are 

 originally impressed, in the same manner as when the pressure 

 varies in the simple ratio of the density. On this account pro- 

 bably, as well as by reason of their great density, liquids and 

 solids are not vehicles so proper for conveying vocal sounds as 

 aeriform fluids. 



Theories of Elastic Fluids. — A few words must now be said 

 on those refined theories respecting elastic fluids, which, pro- 

 ceeding upon certain hypotheses of their ultimate constitution 

 and the action of molecular forces, are directed to the purpose 

 of accounting by mathematical reasoning for certain of their 

 fundamental properties, with which we have originally become 

 acquainted by experience only. Such a theory is that at the 

 commencement of the 12th book of the M^canique Celeste, to 

 which allusion has already been made. The leading principles 

 of this theory are of the following nature. Each molecule of a 

 body, whether in the solid, liquid, or aeriform state, is submitted 

 to the action of three forces : 1°. The repulsion of its caloric by 

 the caloric of the other molecules. 2°. The attraction of its 

 caloric by these molecules. 3°. The attraction of the molecule 

 itself, either by the caloric of these molecules or by the molecules 

 themselves. The caloric of each particle is supposed to be 

 attached to it by the attraction of the particle. In aeriform 

 bodies, the two latter, the attractive forces, are considered to be 

 insensible, and the only action the molecules are subject to is 

 that arising from the mutual repulsion of their caloric. This 

 action is conceived to be independent of the nature of the mole- 

 cules. From these principles Laplace derives, by no very com- 

 plex mathematical reasoning, the fundamental properties of 

 elastic fluids, viz., the law of Mariotte, the law of Dalton and 

 Gay-Lussac, (which are shown to be true of mixed as well as 

 simple gases,) and the law of the pressure of mixed gases. The 

 same principles, together with the consideration of sensible and 



