In a recent article on page 204, this volume SCIENTIFIC AMERICAN, we presented some very useful information on this subject from the London Mechanics' Magazine It was stated from a series of experiments conducted by George Tosh, that brassboiler tubes were found to possess an evaporating power exceeding those of iron twentyfive per cent, and that copper tubes exceeded those of brass thirtyone per cent We observed that the accuracy of the experiments was doubtful, and our doubts we perceive are confirmed by the last issue of the London Artisan, received by us since the article referred to was published It contains a very good report of the discussion which was elicited by the reading of Mr Tosh's paper before the Institution of Mechanical Engineers At that meeting, W B Johnson stated that the results obtained by the author of the paper (Mr Tosh) were very different from his experience, as he had been led to the conclusion that there was no appreciable difference between iron and brass in evaporative power He had a good opportunity of comparing them on a large scale in " two boilers of 160 horse power each, which had been made exactly alike, excepting that one had iron and the other copper tubes The result of the working of these boilers was about equal, and no difference could be noticed beween them" Professor Rankine stated that a series of experiments had been tried a number of years ago, by James R Napier, with experimental boilers of copper and iron of various thicknesses heated over the same gas flame, and he found but a small difference in their evaporative power, about onethirtieth being in favor of copper " In all experiments of the kind," he said, "the state of the heating surface was important, that is, whether smooth or rough, clean or encrusted The effective evaporating result or transmission of heat through metal depends on three propertiesfirst, the resistance of the first surface (that next the fire) to absorption of the heat; second, the resistance of the internal particles of the metal to the conduction of heat; and thirdly, the resistance of the second surface (that next the water) in giving off the heat The resistance to internal conduction is less in copper than iron, but its surface resistance is greater It had been found in experiments very carefully conducted that when the surface became dull, the transmission of heat through all metals was about equal" Mr Siemens stated that Dr Ure had proved by a series of experiments that the conducting power of copper was so good, that by increasing its thickness in a boiler, its evaporative power was not sensibly retarded, while with iron, the result was differentby increasing its thickness, evaporation was greatly retarded On the other hand, Mr Roberts stated he had found that the thickness of the metal in a boilerwhether of copper or iron greatly affected the evaporation of the water The plates, when thick, retarded the passage of heat, and tended to injure the metal by not permitting the caloric to be carried off so rapidly as it should be by the water He foundthat brass tubes of No 18 wire gage, lasted much longer than thicker ones of No/ 14 wire gage, under the same conditions precisely Mr Craig stated he had not found much difference in practice between brass and iron tubes in locomotives, and did not know of any definite result in favor of one more than the other as to evaporative powers Mr Henry Maudsley stated that in steam engine boilersparticularly marine and stationarythere were other reasons affecting the use of copper or iron beside evaporative qualities or conducting power for heat Their durability, under exposure to rusting or corrosion, and liability to encrustations being formed in them, were questions of great importance He had known a case of nine ' marine copper boilers ordered for Naples in A preference to i ron, because allowance had to ,C be made for their being sometimes laid up a without working, and not to suffer from rust, as iron boilers were sometimes under the same conditions seriously injured in eighteen months, while copper boilers were not affected The original cost and conducting power of boilers, under the same circumstances, were secondary questions to durability Mr Tosh then stated that where ho has had charge of locomotives and other engines for several years at Mnryport, he had used a great number of brass and iron boiler tubes with apparently equal success, but brass tubes had been generally preferred for locomotives working at a high pressure, because there is less difficulty i n keeping them fast i n the tube plates, and encrustations are not so liable to form on them as on those of iron ; and when iron tubes became leaky in the least degree, their ends were rapidly destroyed, which was not the case exactly with brass Iron boiler tubes are now extensively employed in England, and many engineers are of opinion that no other kind should be used, but brass is still preferred by the majority The foregoing information on this subject giving the substance of opinions expressed by engineers distinguished in their profession is of much importance, and will interest otir readers generally