248 



HA RD WICKE'S SCIENCE- G OSSIP. 



in the grounds of Dromana House, near Villierstown, 

 in the beautiful valley of the Blackwater, 



In connection with the tenacious vitality of Che- 

 vreul, it is interesting to note that his mother reached 

 the age of ninety-three, and his father died when 

 ninety-one. In most of the well-authenticated cases 

 of long life, it is proved to be common in the family. 

 If this is really a general law, natural selection will 

 increase the average duration of human life as, 

 ceteris paribus, those who live the longest will have 

 the largest families. 



Electrical Absurdities. — A patent has been 

 obtained for a galvanic plant invigorator and oxy- 

 geniser. Two plates or plugs are inserted in the earth 

 on opposite sides of the plant, and connected by a 

 wire. We are told that where the plant is watered 

 " a gentle current of electricity " is produced, which 

 decomposes the water into oxygen and hydrogen, " the 

 former being liberated for the use of the plant." Of 

 the gentleness of the current under such conditions 

 there can be no doubt, but the most rudimentary 

 knowledge of electricity would have taught the in- 

 ventor that such a current cannot decompose water 

 and liberate the gases as stated. Even if the current 

 were strong enough, the liberation of gases would 

 occur only on the surfaces of the plates. 



Another electrical story comes from Germany, 

 telling us that plates of copper were thrust upright in 

 the earth and connected by wires with similarly 

 placed zinc plates, about one hundred feet distant, 

 an electric battery being thus formed, the earth 

 between the copper and zinc completing the circuit. 

 This is said to increase the yield of beets planted 

 between the plates, 15 per cent., and potatoes, 25 per 

 cent. 



Such results revive the papers read at the Royal 

 Society (March 1st, 1880), and at the British As- 

 sociation, 1S81, by the late Sir C. \V. Siemens, in 

 which is described some very marvellous vegetation 

 obtained by using the electric light in a conservatory. 

 The "Times," and some of our popular scientific 

 journals, took up the subject very sensationally, and 

 informed the British public that "it is not unlikely 

 that before many years have come and gone the label 

 ' grown under the electric light ' will be a familiar 

 object in Covent Garden Market, attached not only to 

 grapes, peaches, pine-apples and other kinds of fruit, 

 but also to forced beans, lettuces, tomatoes and other 

 vegetables.'^ Not only have these anticipations failed, 

 but the subsequent well-conducted experiments of 

 M. Deherain (Annales Agronomiques, vol. vii. pp. 

 55 1— 575) have cruelly refuted the more responsible 

 statements of Siemens. 



Mr. James Kennedy, the eminent Lancashire 

 mechanical engineer, has just died at the age of 

 eighty-nine. 



THE TWO MIRRORS. 



By W. J. N. 



No. II. 



WE begin with the Plane Mirror, as its pro- 

 perties are simple and easily understood. 

 Let A B (Fig. 155) represent the section of a plane 

 mirror. Let L D be a ray of light falling upon t 



Fig. 156. 



mirror at the point D, D E be the direction taken by 

 the ray after reflection, and P D be a perpendicular to 

 the surface of the mirror at D, the point of incidence. 

 The angle LDP is called the angle of incidence, and 

 p D E the angle of reflection. These two angles are 

 always equal, the one to the other ; and the incident 

 and reflected rays are in the same plane, which is 

 perpendicular to the surface of the mirror. 



