202 



SCIENCE. 



[Vol. XXII. No. 558 



author of the "new" experiment was totally ignorant. 

 The present publication does not lay claim to any prof ound 

 scientific knowledge or pretend to herald any new 

 discoveries. It is a catalogue of the species of birds known 

 to occur in Michigan, compiled from various published and 

 unpublished data, with notes on localities and other items. 

 There are 332 species recorded. Abstracts are given of 

 bird and game laws, and a bibliography of over 200 

 references adds to the value of the whole. The illustrations, 

 mostly taken from Coues's"Key to North American Birds," 

 will prove of great assistance to those using the Bulletin 

 in the state. J- F- J- 



LIQUID AND SOLID AIE. 



BY JOHN S. MCKAY, PACKER INSTITUTE, BROOKLYN, N. Y. 



The physical state, or condition, of a body is entirely 

 incidental and never dependent upon any inherent prop- 

 erty. The same substance may be solid in one zone and 

 liquid or gaseous in another. According to the kinetic 

 theory, the different states of matter are only different 

 modes of molecular motion and any change of state is the 

 result of the absorption or liberation of energy. By the 

 addition of sufficient heat energy all solids and liquids 

 become gases, and by withdrawing such energy all gases 

 may be reduced to the liquid or solid state. It is proba- 

 ble that at the temperature of absolute zero (- 273''C.) 

 there would be neither solid nor fluid, but that if matter 

 still continued to manifest itself to our senses, it would 

 be in a different physical form from anything now known. 

 It is certain that there could be no gases at that tempera- 

 ture, since molecular motion is essential to the idea of 

 gaseity. From recent experiments it seems probable that 

 all gases, under ordinary atmospheric pressure, would be- 

 come liquid or solid before reaching absolute zero. It is 

 a well-known fact that after a gas has been cooled below 

 its critical -temperature it may be reduced to the liquid 

 state by the aid of external pressure. Until a few years 

 ago oxygen, hydrogen, nitrogen, air, and a few other gases 

 had never been reduced to their critical temperatures and 

 hence could not be liquefied. Air had been compressed 

 until it was denser than water without any trace of lique- 

 faction. And so these gases were called permanent or in- 

 coercible gases. But in 1879 Cailletet of Paris and Pic- 

 tet of Geneva, working independently and by somewhat 

 different methods, succeeded in reaching the critical tem- 

 perature of some of these gases and by great pressure re- 

 duced them to the liquid form. Since then all known 

 gases have been liquefied and the old distinction of per- 

 manent and coercible gases has been effaced. 



The critical temperature, or absolute boiling point, of 

 these gases is very low, being -140°C. for oxygen, -146'^ 

 C. for nitrogen, and -240" C. for hydrogen. This low 

 temperature is obtained by evaporating in vacuo liquid 

 NO,CO,, S0,_ or some other substance whose critical tem- 

 perature is comparatively high and which is therefore 

 easily liquefied. As yet hydrogen has been liquefied only 

 in small quantities by allowing it to expand suddenly 

 when at a low temperature and highly comjjressed. In 

 some remarkable experiments before the Royal Society of 

 London during the past year Prof. Dewar made use of 

 liquid ethylene to secure the low temperature necessary to 

 liquefy air and oxygen. By means of three concentric 

 vessels, the outer one containing liquid nitrous oxide and 

 the next one liquid ethylene, both being connected with 

 powerful vacuum pumps to increase the evaporation, he 

 secured so low a temperature in the inner vessel that oxy- 

 gen, nitrogen and air were liquefied in large quantities 

 with comparatively little pressure. By causing a vacuum 

 to act upon a large tube containing liquid oxygen, a tem- 



perature of -210° C. was produced. A small empty test- 

 tube inserted into the boiling oxygen was so cold that the 

 air of the room at ordinary pressure condensed and 

 trickled down its sides. By evaporating liquid nitrogen 

 in a vacuum, a temperature of -225" C. was reached, at 

 which point nitrogen became solid. 



Liquid oxygen when first formed is milky in appearance, 

 owing to the presence of some impurity which may be re- 

 moved by passing it through ordinary filter paper. When 

 pure it is of a pale blue color, which, however, is not due, 

 as some have thought, to the presence of liquid ozone, 

 which is of a dark blue color. Liquid oxygen is a non- 

 conductor of electricity but is strongly magnetic. It may 

 be lifted from a cup by presenting the poles of a strong 

 electro-magnet. It seems to have very slight chemical 

 activity, since it will extinguish a lighted match and has 

 no action on a piece of phosphorus dropjDed into it. It is 

 well known that the A and B lines of the solar spectrum 

 are due to oxygen, and, from recent experiments on the 

 top of Mount Blanc, it is thought that they are largely if 

 not wholly due to the oxygen in the earth's atmosj)here. 

 Prof. Dewar showed that these lines come out very strong 

 when liquid oxygen is interposed in the path of the rays 

 from an electric lamp. 



Liquid air is at first somewhat opalescent, owing prob- 

 ably to solid particles of carbon dioxide. It may be cleared 

 by filtering or by standing for a few minutes, when the par- 

 ticles rise and disappear. When any of these liquefied 

 gases are placed in an ordinary glass vessel they boil vig- 

 orously and soon disappear owing to the heat obtained 

 from the vessel and surrounding objects. In a vessel 

 made of rock salt they take the spheroidal form and last 

 much longer, but Prof. Dewar found that they could be 

 kept longest in vessels with double walls with high vacua 

 between them. A small bulb filled with liquid air and 

 protected by a vacuum would require an hour and a half 

 to boil away, five times as long as it could be kept in an 

 ordinary vessel. Liquid air has the same higt insulating 

 po ,1 er as oxygen but is less magnetic. Its magnetic 

 power is evidently due to the oxygen, since liquid nitrogen 

 is not magnetic. When the oxygen is attracted by a mag- 

 net it draws the inert nitrogen along with it without being- 

 separated, but if a sponge or ball of cotton be saturated 

 with liquid oxygen and presented to a magnet the liquid 

 will be drawn out of the meshes and cling to the magnet 

 until it evaporates. The normal boiling point of nitrogen 

 is about eight degrees below that of oxygen, so that the 

 two substances may be separated by distillation, the nitro- 

 gan boiling off first and leaving the oxygen. But 

 when air is be ng liquefied the nitrogen does not come 

 down first, as might be expected, but the two condense 

 together at a temj)erature about midway between their 

 respective boiling points. 



All the liquefied gases except oxygen and hydrogen 

 have been frozen by self-evaporation in a vacuum. By 

 evaporating liquid air in a vessel surrounded by liquid 

 oxygen. Prof. Dewar succeeded in reducing the air to a 

 clear, transparent solid. It has not yet been determined 

 whether the oxygen of the mixture is really frozen or 

 merely entangled among the particles of solid nitrogen in 

 some such way as rose ai ater in cold cream, or water in 

 the solid gelatin of calves' foot jelly. Although pure oxy- 

 gen has never been frozen it is j)ossible that wh n mixed 

 with nitrogen its freezing point is raised so that the two 

 solidify together. 



One of the interesting things connected with these re- 

 cent experiments in the liquefaction of gases is the fact 

 that it enables us to j)roduce a lower temperature than 

 ever before. We are slowly creeping down toward the 

 absolute zero and the jjossible solution of the mysteries 

 connected with the nature and constitution of matter. Is 



