This pamphlet was published in 1855 by the American Anti-Slavery Society of New York. It is instructive on a number of levels, but above all for its forthright assertion that whites could be slaves. Not indentured servants, but slaves.
The pamphlet shows that white slavery could happen in various ways, and that the status of slave could be validated in law through maternal heritage. The white slaves in question were considered to be negroes by blood.
But this in itself is interesting. Had white slavery been unknown, there would have been little need for legal disputes and formal tests by which to settle those disputes. Or, put another way, the reality of white slaves must have been familiar enough for antebellum abolitionists to drum it up into a fearsome threat against whites in general (and potentially a motivation for civil war).
Some argue that there is a bright wall of separation between white servitude and slavery, because the one was temporary and contractual whereas the other was lifelong and involuntary. The white “negroes” of the pamphlet show that that wall was easily breached. An example of such a breach would be the famous Louisiana case of Sally Miller: