SCIENCE- G OS SIP. 



CONTRIBUTED BY W. H. CADMAN. 



Density of Ice. — A description is given in the 

 " Phys. Rev." of recent determinations of the 

 density of ice by weighing it in ice-cold water 

 with suitable precautions. The authors find the 

 density of old and new river ice is the same, and 

 may be taken as 0-91661 ± 0-00007. 



Colours op Butterflies. — It is a common 

 error to suppose that butterflies owe their beauty 

 to diffracted light. W. B. Croft has recently 

 examined a collection of British Lepidoptera and 

 found no specimens that were coloured by wave 

 interference. The colour effect is almost entirely 

 due to coloured scales containing pigment. 



Iron Cases for Magnets. — In the "Ann. d- 

 Physik." for August 1901 an account is given of a 

 series of experiments by I. Klemencic upon the 

 protective effect of iron cases for magnets. He 

 observed the moments of three permanent magnets 

 kept in iron cases and found that no change of 

 moment occurred when the magnets in their cases 

 were subjected to shock. The experiments prove 

 the great advantage of keeping magnets in iron 

 cases. 



New System of Signalling at Sea. — Captain 

 Brinkworth, of Gloucester, has invented a new kind 

 of compass-card with the object of assisting fog- 

 bound vessels. The card has a list of sound- 

 signals to correspond with various points of the 

 compass. It is claimed that if this system were 

 adopted a ship in a fog could bellow out, on its 

 fog-horn or whistle, a distinct intimation of the 

 course it is steering. A timely warning would 

 thus be given to any ships in the neighbourhood, 

 and the risk of collisions greatly minimised. All 

 will appreciate this invention who have had the 

 experience of steaming in a thick fog at slow speed 

 with the whistle sounding continuously, and 

 answering signals coming from somewhere at hand, 

 but in what direction it is often difficult to tell. 

 It should prove a boon to captains, if practicable. 



Heat-producing Capabilities of Coal.— The 

 "calorific power" of coal is determined experi- 

 mentally by means of a calorimeter, which consists 

 of a large vessel holding a definite quantity of 

 water. A weighed amount of the coal to be tested 

 is ground to powder and carefully mixed with a 

 composition of saltpetre and potassium chlorate, 

 about 1 part of coal to 10 parts of the mixture. 

 This is placed in a copper cylinder enclosed in a 

 second copper vessel, a fuse is inserted, and when 

 lighted the copper vessels are put in the vessel of 

 water. When combustion ceases a stopcock is 

 opened, which allows the water to fill the copper 

 vessels, and all the heat is absorbed. From the 

 rise in temperature of the water and by means 

 of a table supplied with the instrument the calorific 

 power is determined, either in units of heat or 

 evaporation. 



Measurement of Atmospheric Pressure. — 

 The principle of the Cartesian diver has been put 

 to a new use by G. Guglielmo for the measure- 

 ment of atmospheric pressure and its varia- 

 tions. A great advantage is that it is possible to 

 eliminate temperature effects altogether. The 

 diver may be weighted so as to rise or sink at a 

 certain pressure. By increasing the air space and 

 the density of the liquid the sensitiveness may be 

 increased almost indefinitely. 



Novel Use of the Gramophone. — At the 

 opening of the session of the Eoyal Geographical 

 Society Sir Harry Johnson gave an interesting 

 account of his recent explorations in Central Africa. 

 The languages of the natives, as well as their musi- 

 cal abilities, were demonstrated by means of the 

 gramophone. This instrument should prove a 

 valuable aid in the scientific examination of the 

 Dark Continent. The author is to be congratulated 

 on the introduction of this novelty in the way of 

 lecture illustrations. 



The Pressure of Light. — In the January 

 number of Science-Gossip a short account was 

 given of Professor P. Lebedew's research by means 

 of which he demonstrated experimentally the pres- 

 sure of light. A remarkable case of simultaneous 

 discovery is given in the " Physical Review " for 

 November 1901, in which E. F. Nichols and G. F. 

 Hull describe a preliminary communication on the 

 pressure of heat and light radiation. This account 

 was published almost on the same day as Lebedew's 

 experiments proving the pressure of light. The 

 American physicists used a bolometer of special 

 construction to measure the energy of the radiation 

 causing the pressure ; whereas Lebedew used a 

 calorimeter. In both cases a torsion balance and 

 an arc-lamp were employed. Messrs. Nichols and 

 Hull found that the measured radiation pressure 

 is to the radiation pressure, which the theory 

 applied to the bolometer measurements would re- 

 quire ; as 78 : 100. They point out several sources 

 of error. Professor Lebedew, however, obtained 

 almost perfect agreement. 



Radiation at Low Temperatures. — Many 

 experiments have been made with the object of 

 determining the relation between the rate of cool- 

 ing of a body in a vacuum and the temperature of 

 the body. Dulong, Petit, and Desains used a 

 blackened copper ball, suspended by a thermo- 

 couple in the centre of a glass bulb. The most 

 recent experiments are those made by M. Compan, 

 who carried these experiments down to the 

 temperature of liquid air. The bulb was exhausted 

 by means of a mercury pump. The copper ball 

 was then heated without being removed, by con- 

 centrating upon it the light of an arc lamp. The 

 bulb was placed successively into melting ice and 

 liquid air. Compan found that the formula pro- 

 posed by Stefan, R = E,T 4 , agreed best with the 

 experimental results, the rate of cooling being 

 proportional to the fourth power of the absolute 

 temperature of the cooling body and its envelope. 

 The formulae proposed by other experimenters do 

 not agree with the observed results at low tempera- 

 tures. 



Oil Films on Water. — In connection with the 

 interesting note on this subject in the last issue 

 (ante, p. 256), I may mention that many years 

 ago attempts were made to distinguish between 

 different kinds of oils by the difference in what 



