496 KEPORT — 1894. 



atoms we should expect that there would be an increase in volume. This 

 increase in volume has been observed by Pringsheim with tlie mixed gases, 

 though he ascribes it to another cause. I have undertaken some experi- 

 ments with unmixed chlorine, and I found that in this case there is 

 absolutely no increase in volume. 



Now with regard to the analogy of chemical action and electric 

 discharge we find that light, and light of the same kind as that which 

 brings about chemical action, has a remarkable influence on the passage 

 of the electric discharge. This phenomenon was first noticed by the late 

 Dr. Hertz, who found that when the negative pole of a spark gap was 

 illuminated by an actinic light a discharge would pass which would not 

 pass if the spark gap were unilluminated. It may be that the action of 

 light is to split up molecules into atoms, and that the ready passage of the 

 discharge is thus explainable. However this may be, I think that the 

 analogy of the phenomena of electric discharge and chemical combination 

 may be of importance. I have attempted to point out the analogy which 

 exists when we work with substances which are in an exceptional state of 

 dryness. Whether the analogy holds for other influences which affect 

 chemical combination, and whether the analogies which I have indicated 

 are only superficial and not real, can only be decided by a long series of 

 experiments to which I hope to devote myself. 



Report on Vlanimeters. Bij Professor 0. Henrici, F.R.S. 

 [Ordered bj- the General Committee to be printed in extenso.'] 



Several classifications of planimeters have been used by different writers, 

 and different names have been used to distinguish them. 



Mr. Boys distinguishes three types, which he calls Radius Machines, 

 Sine or Cosine Machines, and Tangent Machines. 



Professor Hele-Shaw has two classes only, according as the recording 

 apparatus does or does not show slipping. 



A more fundamental classification seems to be got on considering first 

 of all the geometrical generation of the area by the motion of a line. This 

 gives us three types : the first follows the generation of an area, as in the 

 Integral Calculus, by rectangular co-ordinates ; the second by polar co- 

 ordinates ; whilst the third is based on purely geometrical considera- 

 tions due to Amsler. 



In the following Report the classification adopted is : — - 



Type I. Orthogonal planimeters. 



J, IT. Polar co-ordinate planimeters. 

 „ III. Planimeters of the Amsler type. 



Instruments of either type may have a recording oi- integrating appa- 

 ratus with or without slipping, and this would give rise to subdivisions, 

 -which, however, it is not necessary to elaborate here. 



This Report covers only a comparatively small pai't of the subject of 

 integrators ; it deals only with planimeters proper. Integrators for such 

 purposes as the continuous registration of work done, integraphs and 

 instruments for the integration of differential equations, harmonic ana- 

 lysers, itc, will not be considered. As it is in these more complicated 

 instruments that the injurious effect of slipping is chiefly noticeable, a 

 description of the various integrating apparatus will not be given. For 



