"Wouldn't it be well," I inquired of Harry, "to try the weight in vacuo? It is possible that, while the specific gravity in air is equal to that of the atmosphere, the specific gravity in vacuo may be zero. The apparition--pray excuse me, Mr. Egerton, if the terms in which I allude to you seem disrespectful, but to call you a ghost would be to prejudge the point at issue--the apparition may have no proper weight of its own at all." "It would be very inconvenient, though," said Harry, "to put the whole apparition under a bell-glass: in fact, we have none big enough. Besides, suppose we were to find that by exhausting the air we got rid of the object altogether, as is very possible, that would awkwardly interfere with the future prosecution of our researches into its nature and properties." "Permit me to make a suggestion," interposed the phantom, "if a person whom you choose to relegate to the neuter gender may be allowed to have a voice in so scientific a question. My friend, the ingenious Mr. Boyle, has lately explained to me the construction of his air-pump, which we saw at one of the Friday evenings at the Royal Institution. It seems to me that your object would be attained if I were to put one hand only on the scale under the bell-glass, and permit the air to be exhausted." "Capital," said Harry: and we got the air-pump in readiness accordingly. The spectre then put his right hand into the scale, and we plumped the bell-glass on top of it. The connecting portion of the arm shone through the severing glass, exactly as though the spectre consisted merely of an immaterial light. In a few minutes the air was exhausted, and the scales remained evenly balanced as before. "This experiment," said Harry judicially, "slightly modifies the opinion which we formed from the preceding one. The specific gravity evidently amounts in itself to nothing, being as air in air, and as vacuum in vacuo. Jot down the result, Jim, will you?" I did so faithfully, and then turning to the spectre I observed, "You mentioned a Mr. Boyle, sir, just now. You allude, I suppose, to the father of chemistry?" "And uncle of the Earl of Cork," replied the apparition, promptly filling up the well-known quotation. "Exactly so. I knew Mr. Boyle slightly during our lifetime, and I have known him intimately ever since he joined the majority." "May I ask, while my friend makes the necessary preparations for the spectrum analysis and the chemical investigation, whether you are in the habit of associating much with--er--well, with other ghosts?" "Oh yes, I see a good deal of society." "Contemporaries of your own, or persons of earlier and later dates?" "Dates really matter very little to us. We may have Socrates and Bacon chatting in the same group. For my own part, I prefer modern society--I may say, the society of the latest arrivals."