744 



SCIENCE 



[N. S. Vol. XXXI. No. 802 



SCIENTIFIC JOURNALS AND ARTICLES 

 The contents of the American Journal of 

 Science for May are as follows : " Contribu- 

 tions to the Geology of the Grand Canyon, 

 Arizona. — The Geology of the Shinumo 

 Area," by L. F. Noble (Part I.) ; " Additions 

 to the Pleistocene Flora of Alabama," by E. 

 W. Berry ; " Application of Potassium Ferri- 

 eyanide in Alkaline Solution to the Estima- 

 tion of Arsenic, Antimony and Tin," by H. 

 E. Palmer ; " New Cystid from the Clinton 

 Formation of Ontario — Lepadoeystis clin- 

 tonensis," by W. A. Parks ; " New Petro- 

 graphic Microscope," by F. E. Wright ; " New 

 Ocular for Use with the Petrographic Micro- 

 scope," by F. E. Wright ; " Behavior of Crys- 

 tals in Light Parallel to an Optic Axis," by C. 

 Travis ; " Some Simple Improvements for a 

 Petrographical Microscope," by A. Johannsen ; 

 " Natural Naphtha from the Province of 

 Santa Clara, Cuba," by C. Eichardson and 

 K. G. Mackenzie ; " Intrusive Granites and 

 Associated Metamorphic Sediments in South- 

 western Ehode Island," by G. F. Loughlin. 



J, 



SPECIAL ARTICLES 



THE CRITICAL SPARK LENGTH 



Removing the condensers from the influence 

 machine in order to avoid strong disruption 

 discharge, the insulated metal sheet referred to 

 in a former communication^ placed between 

 the terminals, separates the positive column 

 from the Faraday dark space. In these two 

 regions the mica wind-mill shows that the air- 

 column is moving in opposite directions. In 

 the dark space Franklin's fluid is carried by 

 convection. The air molecules are over- 

 loaded. They flow from the cathode knob to 

 the plate, to which they deliver their charge. 

 On the positive side of the plate the air mole- 

 cules have everywhere a less than normal 

 charge. Franklin's fluid has been drained 

 out of them and into the anode. The dis- 

 charge here involves a transfer of Frank- 

 lin's fluid (Thomson's corpuscles) from mole- 

 cule to molecule. This operation is attended 



^ April 22, p. 628. 



by luminous effects. Here the convection air 

 current and the electrical discharge are mov- 

 ing in opposite directions. If the metal plate 

 be removed, the opposing air currents will 

 mingle. The length of the Faraday space, 

 where the discharge is mainly by convection 

 will now in general have changed. It be- 

 comes less sharply defined. 



If the anode knob is moved up to the Fara- 

 day dark space, we have the critical spark 

 length when disruptive discharge is feeble on 

 account of small capacity. 



If the knobs are brought nearer together, 

 the positive or luminous discharge surrounds 

 the Faraday region where convection pre- 

 vails. A further decrease in the distance 

 between the knobs increases the cross section 

 of the column where the non-luminous con- 

 vection-transfer occurs. The luminous dis- 

 charge is crowded out into longer arc-like 

 paths. This luminous column is what is 

 usually called the discharge. The air current 

 here forms a return for the convection cur- 

 rents within the Faraday dark space. All of 

 these phenomena have been studied in open 

 air, and photographic evidence will be pre- 

 sented in a paper to be at once published by 

 the Academy of Science of St. Louis. Canal- 

 ray effects obtained when the metal plate is 

 provided with an opening have also been 

 photographed. The Hittorf tube referred to 

 by Thomson" is a most striking illustration of 

 phenomena which are above described. In 

 the shorter branch the dark convection dis- 

 charge involves a transfer of gas molecules 

 which in this case forms, with the gas-flow in 

 the longer branch, a continuous circulation 

 around the circuit of the two branches. 



Francis E. Nipher 



-J THE SAN LUIS valley, COLORADO' 



Popularly the San Luis Valley or park is 

 supposed to be the southernmost one of a 

 chain of four great parks, of which North, 

 Middle and South parks are the others. In 



- " Conduction of Electricity through Gases," 2d 

 ed., p. 443. 



' Published by permission of the director of the 

 United States Geological Survev. 



