274 CARNEGIE INSTITUTION OF WASHINGTON. 



field at once suggests itself. No marked influence of this kind was, however, 

 found in the case of the pure resin. 



One of the most remarkable features of the photographic action produced 

 in the case of wood and metal articles is that it is approximately uniform over 

 the plate, although the magnetic field varies both in magnitude and direction. 

 If the action of the magnetic field were one of controlUng the direction of propa- 

 gation of a radiation or gaseous effusion emitted by the substances, one would 

 expect it to vary over the surface of the plate. The absence of such variation 

 practically limits the nature of the effect to one in which there is direct or 

 indirect stimulation of the activity of emission or production of active gas, 

 diffusion being subsequently relied upon for the uniform distribution of the 

 effect over the plate. 



The effect of very slight temperature changes in modifying the action of metallic 

 articles upon a photographic plate is well known, and one has to remember 

 that an electromagnet becomes appreciably warmed during its excitation. 



The presence of resinous articles does not seem wholly necessary for the 

 production of the apparent magnetic effect. Thus, for example, a number of 

 metal articles, namely, lead, iron, nickel, copper, and brass, were set up over 

 an electromagnet in a vacuum for 14 days, and a similar group was set up in 

 the same vacuum, but in a region shielded from the magnetic field. The first 

 set produced strong impressions, light on a dark ground, while the second set 

 produced no appreciable effect. The experiment was repeated with a perma- 

 nent magnet and an exposure of 21 days, with similar but less pronounced 

 results. 



At the stage of the work recorded in the paper here abstracted, the prelimi- 

 nary conclusion was reached that while a magnetic field or vacuum was not 

 essential to the production of effects of the kind recorded by Mace, the mag- 

 netic field appeared to have an effect in intensifying the action in certain cases. 

 The experiments are being continued with the object of ascertaining whether the 

 primary agency producing the effect, in the case of the electromagnets, for example, 

 is really the magnetic field itself or some other influence accompanying the pro- 

 duction of the magnetic field. 



Preliminary summary showing results of direct comparisons of magnetic-observatory 

 standards during 1915 to 1917 by the Department of Terrestrial Magnetism. 

 J. A. Fleming. 



Table 6 gives results of direct comparisons of magnetic-observatory stand- 

 ards obtained by observers of the Department of Terrestrial Magnetism since 

 those summarized in Table A, page 278, Volume II, Researches of the Depart- 

 ment of Terrestrial Magnetism. East declination and inclination of north 

 end of needle below horizon are regarded as positive. The various footnotes 

 will give the additional explanations required with regard to any particular 

 standards, or as to recent changes in them. (See next page.) 



The radium content of sea-salt specimens, collected on the fourth cruise of the Carnegie. 

 C. W. Hewlett. 



The method adopted for estimating the radium content is that suggested 

 by J. Joly, and used by him for estimating the radium content of rocks. The 

 sample to be tested is heated with fusion mixture in a nickel or platinum 

 boat supported in a suitable tube surrounded by an electric furnace. The 

 expelled gases, after passing through a tube of soda-lime, are collected in a 

 rubber bag, from which they may be subsequently sucked into an ionization 

 chamber, which has been previously exhausted by means of a pump. By 



