April 1 8, 1889] 



NATURE 



;8^ 



absence of the minimum of /t/ at ordinary temperatures, 

 this being due to the fact that hydrogen possesses a very 

 low critical temperature combined with a low critical 

 pressure. For if, instead of taking temperature and 

 pressure in the ordinary units, we take temperature in 

 terms of the critical temperature, and pressure in terms of 

 the critical pressure as units, and then with temperatures 

 as abscissa; and pressures as ordinates construct the 

 curve for the observed minimum points of /t', this curve 

 will be found to be one and the same for all gases. As 

 Wroblewski has shown, it is a continuation of the curve 

 for the vapour pressures at difterent temperatures of the 

 liquefied gases, pre- sure and temperature being e.xpressed 

 in terms of the critical. Drawing this curve by the aid 

 of the observations that have already been made with 

 ethylene, carbon dioxide, methane, and other gases, it 

 is found that the pressure for the minimum point rapidly 

 rises with rising temperature, and reaches a maximum of 

 about 3rr for a temperature of \'a,6. From this point, 

 however, the" pressures decrease as the temperature rises, 

 so that, when the temperature is about 3^, the pressure at 

 which the minimum/z' occurs is TT, or the critical pressure ; 

 and if the temperature be further increased beyond this 

 point the pressure of minimum pv is reduced below 

 the critical, and continues still further to fall as the 

 temperature rises. 



Applying this to hydrogen at a temperature of about 3^, 

 the pressure of minimum pv should be 77, the critical 

 pressure, and this as a matter of fact agrees with the 

 result of the above experiments. For 3^ is approxi- 

 mately — 176° C, at which point the required pressure 

 should be the critical or 133 atmospheres, and observa- 

 tion shows that at - 183' C. this pressure is fourteen 



atmospheres, and as, the temperature being lower, we 

 should expect the pressure to be somewhat higher, this is 

 a very close approximation indeed. At higher tempera- 

 tures the pressure falls below the critical, and this 

 evidently takes place so rapidly that at - 103' C, or for 

 about 5^, the pressure of minimum pv is so small a frac- 

 tion of the critical as to be removed putside the range of 

 observation. The critical pressure itself being low of 

 course assists this process. 



This, then, explains the behaviour of hydrogen on com- 

 pression, and why this behaviour differs from that of the 

 other gases. For a gas must be raised to a temperature 

 of over 3^ before it will act like hydrogen, whereas 

 gases have up to the present only been examined at 

 temperatures not far removed from the critical. 



The very low critical temperature cf hydrogen is 

 remarkable. It confirms the saying of Regnault, that 

 hydrogen at ordinary temperatures is a gas plus qtie par- 

 fait. At the same time it shows that no reliance can be 

 put on Pictet's statement that hydrogen was liquefied at 

 about - 140' C. under a pressure of 360 atmospheres ; 

 and whether the temperature attained in Cailletet's 

 experiments was low enough to actually liquefy the gas 

 must be looked upon as extremely doubtful. At the time 

 of his death Wroblewski was planning experiments for 

 the liquefaction of hydrogen, the only thing necessary to 

 make his work complete. 



By cooling hydrogen under a pressure of 1 10 atmo- 

 spheres to — 213° 8 C. by the evaporation of liquid 

 nitrogen, and then suddenly diminishing the pressure, 

 as low a temperature as - 223° C. was obtained, but the 

 hydrogen still remained in the gaseous state and refused 

 to liquefy. H. Crompton. 



THE MANATEE. 



''PHE Zoological Society have added to their living 

 -*- collection in the Regent's Park a young specimen 

 of the Manatee {Manatus americanits), which those 

 who wish to have an opportunity of inspecting an ex- 

 tremely curious form of Mammalian life should take an 

 early opportunity of visiting. The Manatees belong to 

 the' order Sirenia of naturalists, and are sometimes called 



" herbivorous Cetaceans," although it is, to say the least, 

 very doubtful whether they have any near relationship 

 whatever to the true Whales or order Cetacea. These 

 creatures were abundant in former geological epochs, 

 but since the extermination of the Rhytina, or Steller's 

 Sea-cow, at the latter part of the last century, have only 

 two representatives still living on the earth's surface, viz. 

 the Manatee of America and Africa, and the Dugong of 

 the Indian Ocean. 



As will be seen by our outline sketch, taken from the 

 Zoological Society's Proceedings, the Manatee is shaped 

 more like a fish than an ordinary mammal. It is formed 

 exclusively for aquatic life, and inhabits the estuaries and 

 rivers of the American and African continents, where it 

 passes its time browsing on the plants beneath the surface 

 and adjacent to the banks. It remains mosdy under the 

 water, and only raises its head above the surface to breathe. 



The present specimen, which arrived at the Gardens 

 from Liverpool on March 2, is the second example of 

 this singular form that has been received alive by the 

 Society. The first specimen was acquired in August 1875, 

 and lived about a month in the Regent's Park, where it 



attracted many visitors. When dead, however, it was by 

 no means wasted, as it formed the subject of an excellent 

 article on its anatomy read before the Zoological Society 

 by the late Prof. Garrod, and subsequently published in 

 the tenth volume of the Society's Transactions.^ 



In 1879, a pair of Manatees, received from the Island of 

 Trinidad, lived for several months in the Brighton Aqua- 



' A complete account of the anatomy of the M.inatee wa? prepared by 

 Dr. Murie in 1872, from the examin.ition of a specimen of this .inimal im- 

 ported from Surinam for the Zoological Society, which unfortunately only 

 lived just long enough to reach England. This will be found in the eighth 

 volume of the Zoological Trans.ictions. The attention of residents on the 

 Amazons should be called to the existence in that river of a second species of 

 Manatee {Manatus inung^i/s), discovered by the great Austrian naturalist, 

 Johann Natterer, in 1830, but as yet little known in Europe. 



