S. F. Beckham — Origin of Bitumens. 389 



wave curves of current and electromotive force. It is this : If 

 two or more electromotive forces of different frequencies are 

 impressed upon the resonator circuit and their resonant rises of 

 potential are determined for a given resistance in this circuit, 

 then according to theory the ratio of these rises should remain 

 the same for all other resistances within the limits within 

 which the periodicity of the circuit is practically independent 

 of the ohmic resistance. Accordingly, the resistance of the 

 resonator F, fig. 4, was varied gradually from 100 to 250 ohms 

 (the self-induction of inertia coil in the resonator circuit was 

 about - 75 Henrys) and the resonant rises of potential produced 

 by the fundamental frequencies of the two machines (130 and 

 278 p. p. s.) were carefully determined for each particular resist- 

 ance. The ratio of these rises remained constant to within 

 five per cent but the deviations were now in one direction and 

 now in the other. They were undoubtedly due to the varia- 

 tion in the excitation and the speed of the small machine, both 

 of which depended on the potential of the electric mains of 

 the College plant which, of course, could not be kept very con- 

 stant for so long an interval of time as is necessary for this ex- 

 periment, which was about 15 minutes. 



These preliminary experimental tests demonstrate clearly 

 that a resonator of the type given in fig. l b is quite capable of 

 detecting all the frequencies that may exist in an alternating 

 current wave, that its indications are in good agreement with 

 the theory as far as the fundamental frequency is concerned 

 and that it gives a fairly approximate idea of the relative 

 strength of the harmonics. 



(To be continued.) 



Art. LIY. — On the Origin of Bitumens. A Retrospect; 

 by S. F. Peckham. 



It is now nearly thirty years since I first directed my atten- 

 tion to this subject, and during this long period of time the 

 discussion of the question has proceeded along the same lines 

 that were quite clearly outlined in 1868.* 



The chemical theories, notwithstanding the ability with 

 which Mendeljeff treated this aspect of the question in 1877, f 

 are at present receiving very little attention and will be passed 

 by without further notice. In this connection may properly 



* Proc. American Phil. Soc, x, 445 ; Rep. Geol. Sur. Cal., Geology, II; Appen- 

 dix, pp. 73-90. 



fBull. Soc. Chem. de Paris, i, 501; Buer. d. Deut. Chem. Ges., 1877, p. 229 ; 

 Wag. Ber., 1877, p. 1037. 



