December 25, 1902] 



NATURE 



185 



mission the resources of Egypt were not equal to finding the 

 money. In these circumstances, English enterprise came to 

 the rescue. Sir Ernest Cassel, backed by a financial syndicate, 

 undertook to find the money and Sir John Aird contracted to 

 carry out the work. The capital fund is to be repaid by instal- 

 ments of 166,000/. extending over thirty years, and it is antici- 

 pated that the irrigation will produce a revenue of 400,000/. a 

 year. Sir Benjamin Baker has been the consulting engineer, 

 and the work was carried out under the direction of Mr. 

 Fitzmaurice, lately appointed engineer of the London County 

 Council. The dam has thus been entirely carried out by 

 English enterprise and English capital. 



M. 



THE VELOCITY OF PROPAGATION OF 

 XRA YS. 

 R. BLONDLOT has recently made an experimental 

 determination of the velocity of propagation of X-rays, as 

 a result of which he finds that they travel with the same 

 velocity as light. The full account of the work is published in 

 the Comptes rendus for October 27 and November 3 and 10 

 (vol. cxxxv. pp. 666, 721 and 763), and a translation of the first 

 two papers is given in the Electrician for November 21. As 

 the subject is one of great importance, the following brief abstract 

 of the methods used and the results obtained may be of interest 

 to the readers of Nature. 



The method is based on a principle similar to that of komer s 

 method of determining the velocity of light. The arrangement 

 of the apparatus is shown diagrammatically in Fig. I. B and n' 



Fig. i.— Diagram of M. Blondlot's apparatus. 



represent the terminals of the secondary of an induction coil 

 which are connected to the poles A, a' of a Hertz radiator and 

 to the electrodes E, e' of an X-ray tube. Beneath the Hertz 

 radiator is placed a resonator consisting of a copper wire folded 

 into the shape of a triangle dd'c. The spark gap, c, of this 

 resonator is so placed that it receives the X-rays from the focus 

 tube, but is protected from all other radiation by screens of black 

 paper and an aluminium plate. The oscillator aa' consists of 

 two brass cylinders arranged horizontally in a bottle of vaseline 

 oil. By suitably altering the length of the spark gap, the oscil- 

 lator and the focus tube can be made to work simultaneously. 

 The action is then as follows : — At each current of break, the 

 potential between Eand e' rises sufficiently for the X-ray tube to 

 respond. As the potential continues to rise, a spark passes in 

 the oscillator, and this, withdrawing energy from the focus tube, 

 extinguishes it. By careful adjustment, the spark potential of 

 the exciter can be made only slightly greater than the potential 

 necessary to work the tube, in which case the tube will be 

 extinguished very soon after the beginning of the oscillatory 

 discharge, at the end of a time less than a quarter of the period of 

 the radiator. The electric force at the resonator gap only 

 reaches its maximum after a time equal to half the period of the 

 oscillator ; hence if the X-ray tube is close to the gap, the X- 

 rays having been extinguished prior to this, there can be no action 

 of the tube on the secondary spark. This conclusion is verified 

 by interposing a sheet of lead between tube and gap, when it is 

 found that the spark is not affected. 



Now let the focus tube be kept in the same position and the 

 wires ae and A'E'be lengthened each by the same amount. This 

 has the effect of delaying the extinction of the tube by the time 

 required for the Hertzian waves to traverse this extra length of 

 wire, and consequently the disappearance of the X-rays at the 

 spark gap c is delayed by the same amount. The X-rays can, 

 therefore, act upon the spark, and that they do so is shown by 

 the fact that the interposition of a lead sheet now makes the 

 spark less bright. If, on the other hand, the wires ae and a'e' 

 are kept of constant length and the tube moved farther away 

 from the gap, then the X-rays will experience a retardation 

 equal to the time they take to travel from the tube to the gap. 



NO. 1730, VOL. 67] 



The effect of moving the tube farther off should, therefore, be 

 the same as that of lengthening the wires, and this is again con- 

 firmed by experiment ; the spark grows brighter as the tube is 

 moved away, but if a lead sheet be interposed, the brightness is 

 unaffected by moving the tube. For a certain distance between 

 tube and gap, the X-rays will have at the gap their full 

 intensity during the whole of the time the potential at the gap 

 has an appreciable value ; at this point their effect is a maximum, 

 for increasing the distance diminishes their intensity without in- 

 creasing the time during which they and the electric force act 

 together at the gap. This position of maximum can be found 

 by experiment. 



Let v and v' be the rates of propagation in centimetres per 

 second of the Hertzian waves and the X-rays respectively, and 

 after determining the position of the tube giving the maximum 

 spark when the connecting wires ae and a'e' are of given 

 length, let these wires be lengthened by a cm. The cessation of 

 X-rays at the gap is thus retarded by a/v seconds. In order 

 to re-establish the coincidence of the times and to find the 

 new maximum, the tube must be moved nearer to the gap by a 

 length S cm., such that fj/v' = o/v. The experiment gives $/a, 

 and therefore v'/v. The following table gives the results of a 

 series of experiments, the first column giving the values of a, 

 the second and third columns the values of 3 as determined by 

 M. Blondlot himself and his assistant, M. Virtz, respectively, 

 and the fourth column the mean of these two values. Each of 

 the numbers in columns 2 and 3 is the mean of five deter- 

 minations. 



The mean result of all the experiments in this and other 

 series gives the value C97 for the ratio v'/v. 



A variation of the method was also tried in which the ends 

 of the resonator were separated by 03 cm., and two wires 

 soldered to them and connected to a micrometer spark gap. 

 These wires were bent back on themselves so as to bring the 

 new gap into the same position as the old one. The Hertz waves 

 have to traverse these wires before producing the spark, and if 

 each wire is lengthened by a cm., the spark is retarded by a/v 

 seconds. To obtain the new maximum, the tube must be moved 

 away from the gap by a distance b cm. such that b/v' = a/v. 

 A number of very concordant experiments by this method gave 

 a mean value C93 for the ratio v'/v. 



The final result of all the experiments, therefore, leads to the 

 conclusion that the velocity of propagation of X-rays is equal to 

 that of Hertzian waves or of light through the air. M. Blondlot 

 concludes his papers by pointing out that this conclusion is in 

 harmony either with the hypothesis that X-rays are radiations 

 of very short wave-length or with that of E. Wiechert and Sir 

 George Stokes, that they are electromagnetic impulses produced 

 by the impact between the molecules or electrons in the 

 kathode stream and the antikathode. The fact brought out by 

 these experiments that the X-rays cease simultaneously with the 

 current traversing the Crookes' tube, also supports the latter 

 hypothesis. Maurice Solomon. 



RECENT DIETARY STUDIES. 



'"THE character of the daily menu is influenced by various 

 considerations, but it will be universally conceded that the 

 idiosyncrasies of the palate play the predominant part, and to sug- 

 gest to the ordinary housekeeper that scientific principles should 

 be allowed a voice in the determination of our diet would be 

 simply to court ridicule, for of all departments of the household 

 the kitchen is probably the most conservative in its customs 

 and the most dominated by habit and tradition. It will not be 



