590 



SCIENCE 



[N. S. Vol. XLVI. No. 1198 



The velocity of tlie ions and consequently 

 the time taken for their passage over a given 

 distance may be obtained by measuring the 

 volume of air passing in a given time. A 

 common gas meter {M) provided with a dial 

 one turn of which registered one half of a 

 cubic foot was used in these experiments. The 

 air passing through the tube may be dried by 

 calcium chloride and ions prevented from en- 

 tering with the air stream by a plug of cotton 

 wool placed at (.E). Lead screens (S) should 

 be erected to shield the electroscope from direct 

 radiation. Provided radium salt be used as 

 ionizing agent at a short distance from the 

 electroscope, these screens will need to be sev- 

 eral centimeters thick. 



The gas passing through the tube may be 

 ionized by X-rays or 7-rays shot through a slit 

 {B) cut in the tube and covered by a thin mica 

 sheet, or the ionizing source may be placed in- 

 side the tube. A 2 or 3 mm. spark between 

 the secondaries of an induction coil sealed into 

 the tube provides a convenient source of ioniza- 

 tion for demonstration purposes. X-rays also 

 produce powerful ionization effects. A 2-inch 

 X-ray bulb run at dull luminescence by a coil 

 capable of producing a 4 cm. spark will pro- 

 vide sufficient ionization for the experiments 

 described below. 



A Nernst lamp is more suitable for project- 

 ing the gold leaf on a screen than the ordinary 

 lantern. For laboratory work a low powered 

 microscope with a divided scale in the eye 

 piece is used for measuring the rate of fall of 

 the gold leaf. 



An electroscope of the ts^pe shown in Fig. 1 

 may be made of comparatively small capacity. 

 If the leaf be charged to a relatively high po- 

 tential, it becomes an instrument of high 

 sensibility. Owing to the extremely small 

 mass of the gold leaf it will rapidly alter its 

 rate of deflection as the number of ions swept 

 into the chamber changes. 



Place the X-ray bulb directly over the° elec- 

 troscope and charge the gold leaf, by means of 

 an ebonite rod, till it shows large divergence. 

 Run the bulb for an instant and the gold leaf 

 at once drops a distance proportional to the 

 ionization produced in the electroscope by di- 



rect radiation. If the charging cap is not re- 

 placed before starting the bulb, the sudden 

 drop takes place as before, but the leaf instead 

 of stopping its motion as suddenly as it be- 

 gan gradually slows up with time. This effect 

 is produced by the ions in the air surrounding 

 the electroscope rapidly diffusing into it, the 

 number diminishing as recombination takes 

 place. Some idea of the rapidity with which 

 the leaf comes to rest may be obtained from 

 Table I. The numbers represent readings on 

 the scale between the intervals stated in the 

 first column. The readings with 3 second in- 

 tervals were observed and recorded without as- 

 sistance. Shorter intervals required assistance 

 in making the record. 



Place the X-ray bulb over the slit as indi- 

 cated in Fig. 1 and start the suction pump. 

 When the radiation passes through the slit 

 large quantities of positive and negative ions 

 are produced in the air stream directly be- 

 neath. If the bulb is but a short distance 

 from the electroscope and the air velocity is 

 high, a large proportion of the ions originally 

 produced will be swept into the chamber caus- 

 ing a rapid rate of fall of the gold leaf. The 

 remainder have either recombined or diffused 

 to the side of the tube. Since the negative 

 ions diffuse more rapidly than the positive, the 



