NATURAL PHILOSOPHY. 171 



Shape of Electric Perforations. The curious fact has l^en ob- 

 served, by means of the microscope, that perforations made by the 

 electric spark are uniformly pentagonal in form. 



Point of Absolute Cold. This point or that of deprivation of all 

 heat is estimated to be about 275 below zero Cent., equal to 

 463 below zero F. 



Colors of Metal ivhen Heated. Pouillet measured the temperatures 

 corresponding to the colors which metal takes when heated in a 

 fire, and found them to be as follows: Incipient red, 525 Cent. ; 

 dull red, 700; cherry red, 900; dark orange, 1,100; white, 

 1,300; dazzling white, 1,500. 



New Use for the Sun. Mr. Thomas Boyd, of Cambridgeport, 

 Mass., has perfected a very simple mechanical contrivance by 

 which an efficient system of ventilation can be secured by utilizing 

 the heating power of the sun. It consists essentially of a double 

 cone-shaped iron chamber, placed on the top of a chimney, painted 

 black, in which are set many glass lenses about 2 inches in di- 

 ameter, which concentrate the rays of the sun, and produce within 

 the chamber such a heating and rarefaction of the air, as by its 

 ascensional power to cause a strong upward current. The great 

 advantage of this over revolving ventilators is that it acts most 

 powerfully on still, hot, sunny days, when ventilation is most 

 needed, and when other natural systems fail. In the absence of 

 the sun and at night, the form and mechanical arrangement of the 

 caps make it a superior ventilator. He considers it of great prac- 

 tical importance to have the source of heat so near the place of 

 discharge. Any waste heat from gas-burners or furnaces may also 

 be used in the absence of the sun. It is effectual with any system 

 of heating, and is especially suited for tropical climates. 



Induction Currents. M. Regnault presented to the French 

 Academy, in the name of Prof. Blaserna, of Palermo, results of 

 experiments on the passage of induction currents. His conclu- 

 sions are : 1. The time elapsed between the closing of the rupture 

 of the circuit and the appearance of the current of induction, or 

 the attraction of the armature for the bobbin of induction, is inap- 

 preciable, less than the fiftieth part of a second. 2. The cur- 

 rent of induction, feeble at first, increases little by little, then 

 diminishes, and is extinguished in an interval difficult to deter- 

 mine. 



Magnetic Phantoms. In "Cosmos" for July, 1867, M. Meunier 

 gives the following method for preserving the figures, called 

 " magnetic phantoms," produced when iron filings are scattered 

 on a sheet of paper over a magnet. He prepares the. paper by 

 saturating it with a warm solution of ferroeyanide of potassium, 

 and then thoroughly drying it. The magnetic figure is then made 

 with powdered magnetite or natural loadstone ; then a little pure 

 gaseous hydrochloric acid is allowed to come upon the paper 

 through an inverted funnel ; this is removed after a few seconds, 

 and the paper, freed from the magnetic powder, thoroughly 

 washed. A dark-blue figure of remarkable perfection is thus ob- 

 tained on a faint blue ground. 



New Form of Battery. According to M. Rouillon^s observa- 



