July 1, 1889.] 



KNOW^LEDGE 



193 



but any one who will take the trouble to make the experi- 

 ment with a red-hot poker will be able to see that the effect 

 ))er.si.sts for quite an appreciable time — certainly long enough 

 for a flash of lightning to appear. 



Jt should not be difficult to directly verify the theory, 

 hero advanced for (I believe) the first time. Let some one, 

 who has the requisite appliances, so arrange the beam from 

 an oxyhydrogen or arc lantern, that a series of i=parks from 

 a Wimshurst machine may play athwart it in a suitable 

 atmosphere. The track left by these sparks should, if there 

 be anything in the theoi-y, be readily photographed. — Yours 

 faithfully, 



1 Eoehampton Street, S.W. T. B. Russell. 



[The objection to Mr. Rassell's theory which presents 

 itself, is that the dark flash must be seen through a great 

 thickness of dust-laden illuminated air, and also on a back- 

 ground of dust laden air equally lit up, and a small thick- 

 ness of dustless air in the region of the dark flash would 

 make no appreciable diSerence in the amount of light re- 

 ceived from the direction of the flash. — Editor.] 



THE VANILLA. 



it is produced from a species of orchid. 

 This plant seems to require very little soil 

 for its nourishment, and it generally attaches 

 itself by means of its little aerial rootlets to 

 walls, trees, and other suitable objects. It 

 has a somewhat long and fleshy stem, and the leaves are 

 alternate, oval, and lanceolate (shaped like a lance). The 

 flow<r is of a greenish white, and forms axillary spikes. The 

 fruit, which is a pod, when full grown measures from ten 

 to twelve inches, and is about half an inch in diameter. The 

 commercial vanilla (from the Spanish, vainilla, diminutive of 

 vaina, a pod) is generally produced from the plant Vanilla 

 planifolia (Andrews), a native of Eastern Mexico. It is 

 also extensively cultivated in Eeunion, the Seychelles, and 

 Java, but the Mexican vanilla is t.hought to be the best. 

 The quality of a vanilla pod can always be determined by 

 the presence or the non-presence of a crystalline efflorescence 

 called (jivre, and also by the colour of the pod, which in the 

 best varieties is of a dark chocolate brown. But it is 

 the crystalline efflorescence which contains the sub- 

 stance to which the fragrance of vanilla is due. 

 This substance is called vanillin, and is chemically 

 known by the formula CsHsOa. The pods contain 

 also vanilla acid, oily matter, soft resin, sugar, gum, 

 and oxalate of lime. The choleraic effects that sometimes 

 occur through eating ices flavoured with vanilla may not be 

 due to the vanilla, but to putrefactive changes in the 

 milk ; but it is known that the vanilla plantations are sub- 

 ject to the attack of a little pest known as liacterium pulre- 

 (linin, and it is quite likely that the poisonous effects from 

 ire-eating can be accounted fi>r by the presence of some 

 microscopic fungi in the vanilla. 



In the plantations the vanilla plant is generally fertilised 

 by hand, but, like other orchids, there is no doubt its fertili- 

 sation is promoted by insects wlion in its natural state. 

 The wild plant yields a smaller fruit, and is distinguished in 

 Mexico as Jiai/nilla cimarona, while the cultixated vanilla 

 they call Baynilla corriente. A. J. F. 



REFEACTIOS OF MAGNETIC RADIATION. 

 As the readers of KNOWLf;DGE are aware. Dr. Hertz has 

 made us familiar with the phenomena of reflection and 

 interference of ether waves due to electro-magnetic radia- 

 tion. By means of a prism of pitch, he has more recently 

 obtained an experimental proof of their obedience to the 

 laws of refraction. Professor Oliver Lodge has now gone a 

 step further. In a paper read before the Physical Society 

 he described a series of experiments he had carried out in 

 conjunction with Dr. J. L. Howard, in which they had 

 employed a pair of plano-convex hemi-cylindrical len.ses, 

 made of the best commercial pitch. With these lenses they 

 performed such experiments as would be made with a beam 

 of light and .similar lenses of glass. We may mention that 

 the lenses were 85 centimetres high, 'JO centimetres broad, 

 and 21 centimetres thick. The weight of them being about 

 3 cwt. each. Dr. Lodge found that the results obtained with 

 them were throughout in complete agreement with those of 

 Professor Hertz. 



The Magic LanUrn Journal is the title of a new 

 monthly which has been sent us. The existence of such a 

 periodical may be taken as evidence that there is some one 

 who believes there is a large class interested in the illustra- 

 tion of lectures. We greatly welcome the growing tendency 

 to illustrate lectures by photographs thrown on the screen. 

 If the photographs are untouched, such illustrations bring 

 the audience more directly into contact with the thing to be 

 studied than any hand- drawn pictures. 



The total length of the submarine cables at present in use 

 is given by an Austrian paper at 113,031 miles. Of this 

 length 102,531 miles belong to various companies, and 

 10,500 are Government property. — Electrical Review. 



THE FACE OF THE SKY FOR JULY. 



By Herbert Sadler, F.R.A.S. 

 piERE is .still a marked absence of spots of 

 any size on the sun's surface. During the 

 first three weeks of July there will be no 

 real night in any part of the British 

 Islands. Minima of the Algol-type vari- 

 able ^ Libra> (cf. " Face of the Sky" for 

 April) occur on the 1st at 8h. 24m. p.m. 

 on the 8th at ~h. 58m. p.ji., on the 14th at "h. 32m. p.m. 

 on the 22nd at 7h. 6m. p.m., and on the 28th at 6h. 40m. p.m. 

 Mercury is a morning star throughout July, and is well 

 placed for observation during the second and third weeks of 

 the month. He is at his greatest western elongation on the 

 12th (20° 37' W.), when he rises at 2h. 40m. a.m. or 

 Ih. 20m. before sunrise, with a northern declination of 203=, 

 He appears then as a half moon with an apparent diameter 

 of 7i". On the 20th he rises at 2h. 43m. A.M., or Ih. 26m. 

 before the slin. At 3h. a.m., on the morning of the 9th, 

 Mercury will be about 18' n.p. the A\ magnitude star 

 ■)^ Orionis. During this month he passes from Orion through 

 Gemini into Cancer. Venus is a morning star, and is at 

 her greatest elongation (45.', ) west on the morning of the 

 Kith, when she appears as a half-moon with an apparent 

 diameter of 24". She rises on the 1st of the month at 

 Ih. 28m. A.M., having an apparent diameter of 25.V ", and a 

 northern declination of 15;|*\ t^n the Hist she rises at 

 Ih. Om. A.M., having then an apparent di.imeter of 19" and 

 a northern declination of 20.if°. Throughout July she is in 

 the constellation Taurus; passing thi'ough tlie northern 

 portion of the Ilyades about the middle of the month. On 

 the morning of the 13th Venus will be about 40 n p. the 

 4th magnitude star 8 Tauri, the planet and st;ir forming a 



