Jan. 24, 1889] 



NA TURE 



293 



It is sufficient to state that it is unsatisfactory to our 

 author, who holds that dew on the leaves of plants is 

 (we presume he means occasionally and not universally) 

 derived from the plant itself rather than from condensa- 

 tion from the atmosphere. Dew on growing vegetables 

 is produced by the condensation of the transpired 

 moisture from the plant on its own leaves. This explana- 

 tion is proved by direct experiment, and we are not 

 disposed to deny its truth. It is probable, and, in fact, 

 more than probable, that plants which are giving off 

 large quantities of water into a cold or overcharged 

 atmosphere should have a portion of their own moisture 

 thrown back upon them. This fact is asserted in 

 Marshall Ward's translation of Sachs's " Physiology of 

 Plants," when he says, " Much of the water we find early 

 in the morning on the margins of the leaves of many field 

 and garden plants in the form of large drops, and which 

 are generally taken for drops of dew, is really water 

 excreted from the plants themselves." That the air 

 is really the cooling medium by which the moisture 

 rising from the warm soil or the growing plant is con- 

 densed is no doubt often true. The mist which stretches 

 over the meadows at sundown is moisture condensed in 

 the cool air, and thus becomes precipitated upon vegeta- 

 tion, and not only on vegetation, but upon everything 

 else. Where we think Dr. Stockbridge has overstated his 

 case is when he writes in italics, " The declaration is here 

 made that dew is the condensed exhalation of the plant.'^ 

 The statement is too general, and the assertion has too 

 much of the character of a supposed new discovery on the 

 part of the author. The real facts of the case are, that 

 dew is produced in some cases from condensed exhala- 

 tions from the plant, or from condensed moisture rising 

 from the soil ; but also from the precipitation of moisture 

 from higher sections of the air during the night ; especially 

 when the sky is clear. The collection of water in the 

 form of hoar frost upon leafless trees or lawns must be 

 derived from the condensation of atmospheric moisture 

 upon the tree, or upon the grass, cooled by radiation ; and 

 we have no doubt that grass radiates heat on a moonlight 

 night more rapidly than does bare ground. Dr. Stock- 

 bridge lays too much stress on the fact that the earth is 

 warmer than the air when dew is falling. This>he asks 

 us to believe is fatal to the theory that the earth condenses 

 or can condense atmospheric moisture. The radiating 

 power of the earth is very great, and exceeds that of the 

 air, which, in fact, absorbs and retains much heat which 

 otherwise would be immediately lost in space. Dr. 

 Stockbridge argues that the surface of the earth is in- 

 variably warmer than the air at the dew point, but this is 

 not likely to be the case. Even the temperature of grass 

 land is affirmed to be always warmer than the air, and 

 hence it is contended that in no instance can the earth or 

 vegetation be the condenser. We are disposed to think 

 that observation will throw more light on this point than 

 such experiments as are quoted or were made by Dr. 

 Stockbridge. 



It is probable that dew may be precipitated at times 

 by a colder air on a warmer surface, and at other times 

 by a cold soil or cold expanse of leaf from a warm atmo- 

 sphere. Whether the leaf of a grass or the air above it 

 acts the part of "the cold pitcher" is not always to be 

 predicated, but in either case dew would be the result. 



We may point out that, while Dr. Stockbridge is disposed 

 to assert that the soil is always warmer than the air, other 

 authorities are of opinion that the surface, especially of 

 grass, is colder by many degrees than the air. A thermo- 

 meter laid upon grass would, we believe, recede further and 

 record a lower minimum than one suspended 2 feet above 

 the grass. The freezing of dew on grass during summer 

 nights, which is always an unpleasant sight to gardeners 

 and farmers, appears to be accounted for by radiation 

 of heat from the grass surface, while the substance of the 

 grass cuts off radiated heat from below. The absence of 

 dew under shade also is apparently due to radiation from 

 the earth being checked, and the cooling process of the 

 surface of the earth or its vegetable covering being 

 prevented. 



Dr. Stockbridge's book suffers from careless reading of 

 the proofs. This work, the author tells us, he was com- 

 pelled to depute to others. In one place (p. 183) the 

 word " soil " is evidently used instead of air, thereby 

 reversing the author's obvious meaning, and the word 

 " not " is interpolated, which further confuses the sentence 

 hopelessly. Names of authors quoted are mis-spelt in 

 several cases. The book, especially in the earlier pages, 

 is somewhat bombastic and provincial in its style, 

 and, as we have before stated, many of the earlier 

 pages might have been omitted with advantage. The 

 tone of the writing becomes more modest, precise, and 

 student-like as the author approaches the topics which 

 we are informed upon the title-page he professes. 



John Wrightson. 



OUR BOOK SHELF. 



The Kingdom of Georgia. By Oliver Wardrop. (Lon- 

 don : Sampson Low, 1888.) 

 The author of this work sees no reason why Georgia 

 should not become as popular a resort as Norway or 

 Switzerland. Perhaps he takes rather too sanguine a 

 view of the energy even of the British tourist, but every- 

 one who reads his book will certainly wish to have a 

 chance of visiting the country he describes. A brighter 

 or pleasanter book of the kind we have not seen for many 

 a day. The style is fresh and sparkling, and Mr. Wardrop 

 has the secret of awakening and maintaining the interest 

 of his readers without any attempt at picturesque fine- 

 writing. He conveys a remarkably vivid impression of 

 the splendid natural scenery of Transcaucasia ; but it is 

 in describing the Cieorgian people that he displays most 

 effectively his powers as an observer. He has the warmest 

 appreciatioii of the intelligence, bravery, and generosity of 

 the Georgians, and, in the course of his narrative, the 

 extent to which these ancj other qualities affect their social 

 life is shown with much force and animation. There are 

 valuable chapters on the history, the political condition, 

 and the language and literature of Georgia ; and an 

 excellent bibliography of works relating to the country 

 is given as an appendix. The book is also supplied with 

 good maps and illustrations. 



The British fournal Photographic Almanac yor 1889 

 Edited by J. Tradl Taylor. (London : Henry Green- 

 wood and Co., 1889.) 

 This work, as a compendium of photographic art science, 

 could scarcely be more complete. It contains about 300 

 closely-printed pages, consisting of articles written by 

 men who are eminent in connection with photography in 

 its various and ever-increasing branches. Great ad- 



