304 Royal Society : — 



velocity of the medium at B resolved in the direction B Q due to 

 the source at A is the same as the velocity at A resolved in the 

 direction A P due to the source at B. If the waves observed at 

 A and B are sensibly plane, and if the axes A P, B Q are equally 

 inclined to the waves received, we may, in the above statement, 

 replace " velocities " by " pressures," but not otherwise. 



Suppose, now, that equal double sources face each other, so 

 that the common axis is A B, and let us examine the effect of in- 

 terposing a screen near to A. By the reciprocal theorem, whether 

 there be a screen or not, the velocity at A in direction A B due to 

 B is equal to the velocity at B in direction A B due to A. The 

 waves received at B are approximately plane and perpendicular to 

 A B, so that the relation between the velocity and pressure at B is 

 that proper to a plane wave ; but it is otherwise in the case of the 

 sound received at A. Accordingly the reciprocal theorem does not 

 lead us to expect an equality between the pressures at A and B, on 

 which quantities the behaviour of the sensitive flames depends. 

 On the contrary, it would appear that the pressure at A corre- 

 sponding to the given velocity along A B should be much greater 

 than in the case of a plane wave, and then the relative advantage 

 of the position A would be explained. 



It will be seen that, if the preceding arguments are correct, 

 Prof. Tyndall's experiment does not bear out the conclusions that 

 he has based upon it with respect to the observations of the French 

 Commission at Villejuif and Montlhery. No acoustic clouds could 

 explain the failure of reciprocity then observed ; and the more 

 probable hypothesis that the effect was due to wind is not incon- 

 sistent with the observation that the air (at the surface) was 

 moving in the direction against which the sound was best heard. 



Further experiments on this subject are very desirable. 



" On Supersaturated Saline Solutions." By J. G. Grenfell, B.A., 

 F.G.S. 



In making experiments on the sensitiveness of supersaturated 

 solutions to air and greasy surfaces, I was much annoyed by the 

 solutions so frequently crystallizing on the removal of the cotton- 

 wool, as this necessitated boiling the flask again and waiting till 

 it was cool. I noticed that frequently part of the cotton-wool 

 adhered to the mouth of the flask; and it struck me that, in 

 removing this, some fibres must get detached and fall in, carrying 

 with them in all probability crystals of the salt. I soon convinced 

 myself that this was the case, and that cotton-wool is perhaps the 

 worst material that could be chosen for covering these solutions. 

 I now always use paper or tinfoil ; and I find that these can be 

 removed many times from the same solution without inducing 

 crystallization. I then found that even the most sensitive solu- 

 tions could be taken up in a clean glass tube and dropped on a 

 clean glass plate without crystallizing, and that they will remain 

 liquid exposed to the air for a very long time, often, in fact, till 

 they dry up by evaporation in modified forms. Twenty drops on 

 a plate give twenty experiments on the effect of air, clean and 



