Makch 15, 1895.] 



SCIENCE. 



301 



curately determiuable than cross-section. 

 Another somewhat serious mistake, since 

 it is fundamental, is the statement on page 

 18, that specific resistance is ' the resistance 

 of any particular substance as compared 

 with the resistance of a piece of some other 

 conductor, such as silver, both being of unit 

 dimensions,' As a matter of fact, sijecific 

 resistance, which is a very important term, 

 is the resistance in ohms of a unit volume, 

 and is entirely independent of any particu- 

 lar standard substance. The use of the 

 term ' magnetic resistance,' on pages 219 to 

 221, is open to objection, since the term 

 ' reluctance ' is now almost universally em- 

 ployed to distinguish this quantity fi-om 

 electi-ical resistance. 



Taken as a whole, however, the errors 

 are not numerous, and the work is recom- 

 mended as a text or reference book for 

 those who desire to learn the principles, 

 general construction and action of the 

 various kinds of electrical machinery and 

 instruments, with the exceptions already 

 noted. F. B. Ckocker. 



PHYSIOLOGICAL PHYSICS, 



On the Spontaneoiw Heating and Ignition of 

 Hay. Beethelot. Ann. Chim. Phys., 

 7, 2. p. 430. 1894. 



The author finds that poorly dried hay 

 may ignite when the rise in temperature is 

 only to 140° C. (280° Fh.). The evolution 

 of heat necessarj'^ for this rise of temperature 

 is due to the absorption of oxygen in .spite 

 of the interrupted sprouting, which will only 

 take place when the hay is quite wet. The 

 chemical process involving this absoi-ption 

 of oxygen maj' continue until the hay is 

 thoroughly di-y, 



Druck unci Arbeit^letsfung durch Wachiiende 

 Pfianzen. W, Pfekfer. Abh, d, Math,- 

 Phys. Kl, der K. Sachsicher Gesellschaft 

 der Wiss,, 20, p. 235, 1893. 

 Mr. Pfefter investigated very carefully 



and ingeniously the pressure exerted by 



parts of plants in growth, and found, for 

 example, that a root point conld exert a 

 pressure of 10-15 atmospheres. He ascribes 

 these forces to osmotic pressure, and criti- 

 cises the view concerning the growth of the 

 cell-wall, which ascribes it to simple plastic 

 expansion. 



La Lumih-e Physiologique. R. Dubois Rev, 



gen, des Sciences, .5, p, 41.5 and p, .529, 



1894, 



Part first contains a review of light emit- 

 ting organisms, and a description of the 

 organs involved. In part second the author 

 treats the subject of the emission more 

 thoroughly, describing the character of the 

 light radiated, and finds that the brightest 

 PjTophorus i-adiates 1, 4X10-' calorie in 

 ten minutes. 



The author summarizes his extensive in- 

 vestigations as follows : 



Neither a perfect organ nor a perfect cell 

 is necessary for the coming and going of the 

 light. The cell produces the photogenic 

 substance which, once formed, may light or 

 not, according to the conditions surround- 

 ing it. 



They must fulfill the conditions neces.sary 

 for life, must contain oxygen and water, 

 and have a suitable temperature. 



The light (luminous energj') is found to 

 be 90% of the total energy radiated, 



Dubois made a fluorescent substance from 

 the blood of Pyrophorus, which, like that 

 from the animal itself, lost its peculiar prop- 

 erty on being treated with weak acetic 

 acid and regained it on treatment with am- 

 monia. 



All the causes which excite or desti'oy 

 the activitj' of the protoplasm have a simi- 

 lar effect upon the production of the physio- 

 logical light. 



The production of light depends upon the 

 change of living protoplasmic granulations 

 into the condition of lifeless crystalline 

 matter. 



