April, i92I. J CURRENT TOPICS. 569

substances examined. Amyl thioether (C~HnS) gave the high- est odor with the smallest concentrat ion, only o.4 mg. per cubic foot being sufficient to give s t rong intensity. Judging by market price alone, the two cheapest stenches which impart a s trong odor are amyl acetate and ni trobenzene, but both of these are unsat is factory because the odors are not unpleasant to many per- sons. E thy i hydrosulphide (ethyl mercaptan, C2H~HS) and pyridin seem to be the only other stenches with highly disagree- able odors that are quoted at prices that seem wor thy to be con- sidered for large-scale use. Pyridin is especially suitable, for a l though the pure article is quoted at $2.5o per gallon, an impure article, which serves as well, can be obtained from the acid wash- ings of light oils. Phenyl isocyanide has an ext remely disagree- able odor, and, though not listed as a commercial chemical, can be prepared by comparat ively simple and inexpensive methods. Conservative figuring shows that gas can be probably satisfac- torily impregnated with it at a cost of less than one cent per thousand cubic feet.

In view of the s t rong trend to electric supply for l ighting and power, with the consequent relegation of gas to heating purposes, and, therefore, the low enr ichment of it, the use of some powerful stench seems to be a necessity for the protection of life and property.

H . L .

The Ul tra-micrometer ; an Appl icat ion of the Thermionic Va lve to the Measurement of Very Small Dis tances . R. W m - I)IXGTON. (Phil. Mag.. November, t 92o . ) - - In a circuit containing inductance and capacity the f requency of the electrical oscillation natural to such a combination is dependent on the product of the inductance and capacity. If either of these varies, then the fre- quency varies also. Suppose the capacity is in the form of a con- denser consist ing of two parallel plate~. A change in the distance between these causes a change in capacity and this in turn brings about a change in the f requency of oscillation. The author cal- culates that, one- thousandth of an inch being the distance sepa- rat ing the plates, a change of one vibration per second will be brought about by al tering this distance by two billionths of an inch - -and such a change in f requency can actually be detected.

To realize as far as may be the possibilities indicated by theory the fol lowing ar rangement was devised: An oscillating valve circuit was set up, including the parallel plate condenser in which change in the interval between the plates was to be inves- t igated and also a loud-speaking telephone joined to a three-stage t ransformer amplifier. This apparatus gave a f requency of about one million per second. To make evident any change in this f requency a second valve circuit was placed near the first. I ts f requency could be adjusted by means of a condenser so as to nearly equal that of the first. JThus the telephone was caused to

570 CURRENT TOPICS. [J. F. I.

emit a loud, audible note, whose pitch could be studied aga ins t the background of ano ther note main ta ined at a s tandard pitch. The pitch of the te lephone note was tuned to this pitch. Le t the dis tance be tween the plates in the first circuit change. A change in t h e pi tch of the note coming f rom the te lephone follows, and in consequence there is a difference in the n u m b e r of beats heard in a second. A careful s tudy showed that in pract ice a l ter ing the in terval be tween the plates by one two-hundred-mi l l ion th pa r t of an inch caused a percept ible change in the n u m b e r of beats per second. The two notes were b rough t back into unison by m o v i n g the pointer in a var iable condenser in the second circuit. W h e n the distance be tween the plates in the first circuit was made to change by a re la t ively large distance, such as one one-hundred- thousand th of an inch, it was found tha t shif t ing the condenser pointer one- tenth of a degree compensa ted for separa t ing the plates abou t three ten-mil l ionths of an inch.

The method is so sensi t ive tha t the placing of a Bri t ish penny on a s tout bench suppor t ing the appara tus caused a percdptible audible change, because it bent the bench, t i l ted the suppor t s and changed the dis tance f rom one condenser plate to the other. In view of the fact tha t the wave- l eng th of the shor tes t waves of visible l ight is about one s ix ty - thousand th of an inch it is seen that this new method of linear m e a s u r e m e n t can de te rmine dis- tances ve ry much smal ler than the wave- l eng th of l ight ; indeed, it is appl icable to lengths as small as the d iameter of an atom.

G. F. S.

N e w Foods at a B a n q u e t . - - T h e farewell banque t on F e b r u a r y I6th tendered to Secre ta ry Meredi th by the scientific workers of the D e p a r t m e n t of Agricul ture , was marked by inclusion in the menu of several art icles of food tha t have been made avai lable by the labors of the depar tment . T w o of these articles were mate- rials, for the p repara t ion of which pa ten ts have been issued to their devisors, but laid open to public use. These are the " per- fect bread," the resul t of studies by Dr. Carl O. Johns and A. J. Fink, be ing the first food p roduc t furnishing a comple te ly bal- anced rat ion, and candies made fl-om sweet -pota to sirup. T h e formula for the la t ter i tem has been pa ten ted by Doc to r Gore. O the r i tems of the menu were Amer i can roquefor t cheese, sara- toga chips made f rom the newly in t roduced vegetable , dasheen, a new soy-bean sauce developed by Doc to r Church and new varie- ties of grapes, impor ted under the direction of Doc to r H u s m a n n .

H . L .

Cellulose Ace ta te a n d Artificial Silk. (The Chemical Age.)-- Cellulose acetate , f rom which dope for aeroplanes and airships is made, has proved itself to be of the u t m os t value in the aircraf t industry, and the concept ion of laying down and comple t ing the