adopting Ricardo's conclusions, as though they were axiomatic, he asserts that the measure of exchange between one class of commodities and another--such, for example, as cigars, printed books, and chronometers--is the amount of manual labour, estimated in terms of time, which is on an average necessary to the production of each of them. His meaning in this respect is illustrated with pictorial vividness by his teaching with regard to the form in which the measure of exchange should embody itself. This, he said, ought not to be gold or silver, but "labour-certificates," which would indicate that whoever possessed them had laboured for so many hours in producing no matter what, and which would purchase anything else, or any quantity of anything else, representing an equal expenditure of labour of any other kind. Having thus settled, as it seemed to him beyond dispute, that manual labour, estimated in terms of time, is the sole source and measure of economic values or of wealth, Marx goes on to point out that, by the improvement of industrial methods, labour in the modern world has been growing more and more productive, so that each labour-hour results in an increased yield of commodities. Thus a man who a couple of centuries ago could have only just kept himself alive by the products of his entire labour-day, can now keep himself alive by the products of half or a quarter of it. The products of the remainder of his labour-day are what Marx called a "surplus value," meaning by this phrase all that output of wealth which is beyond what is practically necessary to keep the labourer alive. But what, he asks, becomes of this surplus? Does it go to the labourers who have produced it? No, he replies. On the contrary, as fast as it is produced, it is abstracted from the labourer in a manner, which he goes on to analyse, by the capitalist. Marx here advances to the second stage of his argument. Capital, as he conceives of it, is the tools or instruments of production; and modern capital for him means those vast aggregates of machinery by the use of which in most industries the earlier implements have been displaced. Now, here, says Marx, the capitalist is sure to interpose with the objection that the increased output of wealth is due, not to labour, but to the machinery, and that the labourer, as such, has consequently no claim on it. But to this objection Marx is ready with the following answer--that the machinery itself is nothing but past labour in disguise. It is past labour crystallised, or embodied in an external form, and used by present labour to assist itself in its own operations. Every wheel, crank, and connecting-rod, every rivet in every boiler, owes its shape and its place to labour, and labour only. Labour, therefore--the labour of the average multitude--remains the sole agent in the production of wealth, after all.