“We get our repulsive force from the gravity of a heavy body,” answered Dennis; “and in the old days when the ship was first used, the inventors could not control a greater attractive or repulsive force than the gravity of the object from which they obtained it; but that was long ago, and since then science has made great strides. Adding the science of to-day to the secret of the ship’s power, we can get a force equal to the force of the gravity of any particular source multiplied some thousands of times, which makes the _Regina’s_ power irresistible. For instance, we could exert more than a hundred thousand times the power of Jupiter’s gravity, or the sun’s, and could displace both if we wished.” “I should just like to see the sun go the other way round,” remarked Godfrey, musingly. “Would it make much difference?” and as the trio laughed, he continued, “Here, Gilbert, you’re the physicist! Give me some particulars about this heat business, so that I can be thinking things over by the time we get to Jupiter, to enable me to recognise this fire-eating grub when I come across him. Give me his life-history if you can; it will save a lot of trouble.” They all laughed, and Gilbert replied,—“You’ve got to find all that out for yourself, old fellow; we know nothing more than you know already.” “But what _is_ heat? What temperature has the cocoon to stand, and how and when and all the rest of it? You see, I’m working in the dark. Is it heat as matter it must stand? And what is the effect of heat in non-atmospheric space?” “That’s a big order,” responded Gilbert. “To begin with, until we get the web we cannot tell how heat will affect it. As for what _is_ heat it is difficult to say. We cannot take touch as a criterion, as we might say a certain substance ‘feels’ hot or cold, such as wool being classed amongst the hot and metal amongst the cold. Some scientists say heat is ‘ponderable’ and others consider it ‘caloric’—a form of ‘matter,’ but to me both are wrong.” “How do you make that out?” queried Godfrey. “The fact that it is _im_ponderable is fully proved in that it cannot be weighed, for it is well known that a cold substance does not increase its weight on receiving heat, but remains the same weight as before being heated, and it cannot by any possibility be considered ‘matter’ or its ‘quantity’ would remain unchangeable so far as human means could influence it.” “How can that be?” “Because there are innumerable instances in which heat can be and is regularly produced without either flame or combustion, such as raising the temperature by friction, and you know that if several materials of different degrees of heat are placed in the same room they will all become eventually of the same temperature; thus, if a bucketful of iced water is placed in a hot room it will itself be warmed and the air in the room cooled till both are equal. This, therefore, disproves the ‘materiality’ of heat.” “But the laws of heat are constant, are they not?”