1865.] Physics. 495 



but excluded from air and light they preserve their greenness for an 

 indefinite time. In the course ofthe decoloration by solar light, he 

 proves that oxygen was absorbed and carbonic acid evolved. Under 

 the same circumstances he finds the yellow matter in etiolated leaves 

 to absorb oxygen. On the contrary, it is proved that etiolated leaves, 

 in becoming green in the light, evolve oxygen, while those kept in the 

 dark and unchanged in colour absorb oxygen. The author is dis- 

 posed to regard chlorophyll as a body quite distinct from the yellow 

 colouring matter of leaves. 



The magnesium light promises to be of important use for lecturers 

 and physical experimentalists. M. Lallemand, of Versailles, has 

 found that it is sufficiently active to determine the combination of 

 hydrogen and chlorine. The explosion may be effected with the light 

 of a single wire burnt in a spirit lamp. 



Whilst referring to the explosion of hydrogen and chlorine it may 

 be of interest to many of our readers if we give in detail the mode 

 of preparing the sealed bulbs containing exactly equal volumes of 

 chlorine and hydrogen gases, which have been employed by Dr. 

 Roscoe for exhibiting the chemical combination of these gases effected 

 by the action of light. The process was fully described by Dr. 

 Eoscoe himself at one of the recent meetings of the Manchester 

 Literary and Philosophical Society. The apparatus needed consists of 

 a stout tube or narrow bottle of about 120 cubic centimetres capacity, 

 fitted with a caoutchouc stopper with three holes bored through it. 

 Into one of these holes a vent gas delivery tube passes, on to which 

 three small wash bulbs are blown ; into the other two holes are in- 

 serted the rounded ends of two lengths of the gas carbon, commonly 

 used as terminals for the electric lamp ; these poles are of such a 

 length that they pass to the bottom of the glass bottle. This is then 

 filled with strong aqueous hydrochloric acid, containing about 30 per 

 cent, of the anhydrous acid ; the stopper in the poles and wash bulbs 

 containing a few drops of water is then fixed into its position, and the 

 evolution vessel placed in a beaker of cold water, whilst contact is 

 made with the terminals of four ordinary-sized Bunsen's cells, the 

 whole apparatus being placed in a dark room. The mixed gases at 

 once begin to be given off, and ought to pass through the wash bulbs 

 at the rate of about two bubbles per second. It is absolutely neces- 

 sary that the gas be allowed to come off at this rate for three hours 

 before it is collected, as up to this time it does not attain a sufficient 

 degree of purity and sensitiveness, whilst after the lapse of this time 

 it is generally found to be fit for use. In order to absorb the excess 

 of chlorine, the waste gas may be led into a condenser containing 

 slacked lime and charcoal in alternate layers. When the evolution 

 has gone on for the above-mentioned time, a bulb tube, connected by 

 caoutchouc joinings, is placed between the evolution vessel and the 

 condenser, and the gas allowed to pass through. The bulbs, which 

 are made of fusible glass tubing, are blown about the size of a hen's 

 egg, and so thin that they easily break when pressed with the finger. 

 At each side of the bulb the tube is drawn out so as to be very thin 



