If you’ve got a bottle of Hennessy cognac form 1870 I would say that it is worth quite a bit of money! How much? Well as any one who collects things will tell you - it’s all about desirability.
Collectors will probably want to buy it for an investment - some are building a collection - so you really need to find someone who is looking for Hennes. Hennessy is a very well known brand and won’t hold its price as well as some of the lesser brands who have limited editions.
Some collectors are looking for a particular year. If 1870 was significant in any way it will be worth more. Maybe a collector is looking for particular bottle shape or label.
This bottle isfrom the 1860's and you would have to pay about £5000 for it!
If you know any history of the Hennessy cognac (its provenance) that is useful.
Hennessy themselves never get into valuing old bottles. From their website:-
We are absolutely unable to give a price assessment. Only the organizations specializing in auctions of old bottles should be consulted for this sort of item.
On cognac-expert.com they have a blog and on there is a facility to submit pictures and further information if you want a valuation.
I think you could be looking at between £2,000 - £4000 or more depending on authenticity/provenance etc.
Of course you should get an expert valuation and if its worth a lot of money it will pay you to speak to someone who can give you the best advice.
...or you could just drink it with some good friends and have a lovely time! Cheers!