Long stretches of Marx's Capital take the form of an effort at developing and defending an economic model of capitalism, based on the theories of value and surplus value. But there are also recurring efforts at providing a descriptive sociology of capitalism: the forms of day-to-day life that British economic relations imposed upon the working class. This dimension of the book is descriptive and detailed; it has much in common with Engels's approach in The Condition of the Working Class in England.
Marx was very interested in these descriptive investigations -- Dr. Simon, Dr. Julian Hunter, Mr. Smith, Dr. Bell, and the inquiries and Acts of Parliament in the 1860s that shed light on the depth of English poverty. The index for Capital includes a section, "Parliamentary Reports and Other Official Publications," which includes references to over a hundred reports on factories, poverty, nutrition, and health. These range from a Report of Select Committee, London, 1855, on "Adulteration of Bread", to "Reports of the Medical Officer of the Privy Council on Public Health" (1861-66). And these reports constitute the core of empirical evidence that Marx brings to bear for his economic assertions throughout the work. In fact, we might describe some parts of Capital as a sort of "meta-study" of current investigations of the public health status of England's cities.
This interest is particularly evident late in Capital where Marx turns to the topic of "The General Law of Capitalist Accumulation." Consider one fairly detailed section, "The Badly Paid Strata of the British Industrial Class" (link). Here Marx is offering the best information available, at the household level, concerning the standard of living of this stratum of the British working class. Consider this passage:
During the cotton famine of 1862, Dr. Smith was charged by the Privy Council with an inquiry into the conditions of nourishment of the distressed operatives in Lancashire and Cheshire. His observations during many preceding years had led him to the conclusion that “to avert starvation diseases,” the daily food of an average woman ought to contain at least 3,900 grains of carbon with 180 grains of nitrogen; the daily food of an average man, at least 4,300 grains of carbon with 200 grains of nitrogen; for women, about the same quantity of nutritive elements as are contained in 2 lbs. of good wheaten bread, for men 1/9 more; for the weekly average of adult men and women, at least 28,600 grains of carbon and 1,330 grains of nitrogen. His calculation was practically confirmed in a surprising manner by its agreement with the miserable quantity of nourishment to which want had forced down the consumption of the cotton operatives. This was, in December, 1862, 29,211 grains of carbon, and 1,295 grains of nitrogen weekly.Marx then quotes a Privy Council inquiry in 1863, which finds that
"in only one of the examined classes of in-door operatives did the average nitrogen-supply just exceed, while in another it nearly reached, the estimated standard of bare sufficiency [i.e., sufficient to avert starvation diseases], and that in two classes there was defect — in one, a very large defect — of both nitrogen and carbon. Moreover, as regards the examined families of the agricultural population, it appeared that more than a fifth were with less than the estimated sufficiency of carbonaceous food, that more than one-third were with less than the estimated sufficiency of nitrogenous food, and that in three counties (Berkshire, Oxfordshire, and Somersetshire), insufficiency of nitrogenous food was the average local diet.”In other words, the Privy Council finds that important segments of the English working class were undernourished by the prevailing scientific standard of the day. This official report is fundamentally damning of the current economic system; it led to conditions of near-starvation for its workers.
Marx goes on to describe other components of the standard of living -- housing, crowding, sanitation, and clothing, for which the working class are seriously deprived. And in each case he believes that independent, disinterested observers have documented the conditions of misery in which the working class lived in England in the 1860s.
Consider another interesting example, Marx's use of the Select Committee's report on the adulteration of bread. This occurs in an extended footnote in Chapter VI, "The Buying and Selling of Labour-Power."
In London there are two sorts of bakers, the "full priced," who sell bread at its full value, and the "undersellers," who sell it under its value. The latter class comprises more than three-fourths of the total number of bakers.... The undersellers, almost without exception, sell bread adulterated with alum, soap, pearl ashes, chalk, Derbyshire stonedust, and such like agreeable nourishing and wholesome ingredients. (note 3)
Marx goes on to describe in a little bit of detail the specific timing of the payment of wages, demonstrating the economic coercion that leads workers to agree to buy this adulterated bread.
Or we might notice that Marx's index refers to a series of Parliamentary reports on child labor, and then consider the use that Marx makes of these reports. Here is one example of his use of the child labor reports in the chapter on "The Working-Day":
The potteries of Staffordshire have, during the last 22 years, been the subject of three parliamentary inquiries.... For my purpose it is enough to take, from the reports of 1860 and 1863, some depositions of the exploited children themselves. ... William Wood, 9 years old, was 7 years and 10 months when he began to work. He "ran moulds" (carried ready-moulded articles into the drying-room, afterwards bringing back the empty mould) from the beginning. He came to work every day in the week at 6 a.m., and left off about 9 p.m. "I work till 9 o'clock at night six days in the week." (Capital I, chap 7, sect. 3)
What is of special interest in these passages is the way that Marx's mind seems to have worked on these questions. He was genuinely interested, it would seem, in the concrete details of the conditions of the English working class; and he was fortunate that there was something of an explosion of official and non-official interest in the same set of questions, including especially Parliamentary Blue Books. These inquiries fall in the category of what we would today call the field of public health, and Marx was plainly an avid reader of the reports that resulted from these investigations.
It is not accident that brings Marx's detailed discussion of the nutritional status of the industrial poor into Part VII of Capital, "Accumulation." It is Marx's view that the production of surplus value, and the accumulation of wealth that it enables, is directly related to the production of the poverty of the worker. So the circumstances that Marx describes in this section do not constitute simply an unfortunate current reality; they are the apotheosis of a system of production that was working well.