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The policeman's vehicle was standing outside. It was a small, single-seat, two wheeled affair. Having become familiar with the principles of these hydro-carbon engines from examination of the vehicle of the farm, and accustomed as he was to far more complex mechanisms than this crude affair, Hradzka could see at a glance how to operate it.
No matter much what we eat, the system appropriates what elements it wants. The taste bulbs were planted in our mouths for a useful purpose. Our taste is about the surest index to the body's requirements in the matter of nourishment. If our appetite calls for a thing and it tastes all right, it will do us good whether it be carbo-hydrate or hydro-carbon or something else.
Had simple incandescence by heat been in question, the effect would have been different; the two spectra would have been superposed without prejudice to either. Similarly, the replacement of the hydro-carbon bands in the spectrum of the comet by the sodium line proved electricity to be the exciting agent.
According to Bredichin, the straight tail must have been composed of hydrogen, and the other of some form of hydro-carbon whose atoms are heavier than those of hydrogen, and, consequently, when swept away by the storm of light-waves, followed a curvature depending upon the resultant of the forces operating upon them.
It is true that a second noticeably curved tail was seen in Chili, March 1, and at Madras, March 11, 1843; and the conjecture was accordingly hazarded that the materials composing on that occasion the principal appendage having become exhausted, those of the secondary one remained predominant, and reappeared alone in the "hydro-carbon" train of 1880.
Other materials are also present; but the hydro-carbon element is probably unfailing and predominant. Its luminosity is, there is little doubt, an effect of electrical excitement.
On comparing its spectrum with that of an olefiant-gas "vacuum tube" rendered luminous by electricity, he found the agreement exact. It has since been abundantly confirmed. All the eighteen comets tested by light analysis, between 1868 and 1880, showed the typical hydro-carbon spectrum common to the whole group of those compounds, but probably due immediately to the presence of acetylene.
At Potsdam, on June 30, the hydro-carbon bands were indeed traced by Vogel to the very end of the tail; and they were kept in sight by Young at a greater distance from the nucleus than the more equably dispersed light. There seems little doubt that, as in the solar corona, the relative strength of the two orders of spectra is subject to fluctuations.
Father Secchi, in the clear sky of Rome, was able to push the identification even closer than had heretofore been done. The complete hydro-carbon spectrum consists of five zones of variously coloured light. Three of these only the three central ones had till then been obtained from comets; owing, it was supposed, to their temperature not being high enough to develop the others.
Among the particles of hydro-carbon, and perhaps solid carbon in the state of fine dust, which are thus set free there will be many whose size is within the critical limit which enables the light-waves from the sun to drive them away. Clouds of such particles, then, will stream off behind the advancing comet, producing the appearance of a tail.
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